Expression System for Recombinant Human Growth Hormone Production
from Bacillus subtilis
¸ ¨ ¨ ¨ ¨
Tuncer H. Ozdamar, Birgul S enturk, Ozge Deniz Yilmaz, and Guzide Calık
Biochemical Reaction Engineering Laboratory, Chemical Engineering Dept., Ankara University, 06100 Ankara, Turkey
Eda Celik and Pınar Calık
Industrial Biotechnology and Metabolic Engineering Laboratory, Chemical Engineering Dept., Middle East Technical University,
06531 Ankara, Turkey
Published online January 8, 2009 in Wiley InterScience (www.interscience.wiley.com).
We demonstrate for the ﬁrst time, an expression system mimicking serine alkaline protease
synthesis and secretion, producing native form of human growth hormone (hGH) from Bacil-
lus subtilis. A hybrid-gene of two DNA fragments, i.e., signal (pre-) DNA sequence of B.
licheniformis serine alkaline protease gene (subC) and cDNA encoding hGH, were cloned
into pMK4 and expressed under deg-promoter in B. subtilis. Recombinant-hGH (rhGH) pro-
duced by B. subtilis carrying pMK4::pre(subC)::hGH was secreted. N-terminal sequence
and mass spectrometry analyses of rhGH conﬁrm the mature hGH sequence, and indicate
that the signal peptide was properly processed by B. subtilis signal-peptidase. The highest
rhGH concentration was obtained at t ¼ 32 h as CrhGH ¼ 70 mg LÀ1 with a product yield
on substrate YrhGH/S ¼ 9 g kgÀ1, in a glucose based deﬁned medium. Fermentation charac-
teristics and inﬂuence of hGH gene on the rhGH production were investigated by comparing
B. subtilis carrying pMK4::pre(subC)::hGH with that of carrying merely pMK4. Excreted
organic-acid concentrations were higher by B. subtilis carrying pMK4::pre(subC)::hGH,
whereas excreted amino-acid concentrations were higher by B. subtilis carrying pMK4. The
approach developed is expected to be applicable to the design of expression systems for het-
erologous protein production from Bacillus species. V 2009 American Institute of Chemical
Engineers Biotechnol. Prog., 25: 75–84, 2009
Keywords: Bacillus, recombinant, protein, human growth hormone, degQ, signal peptide,
expression, secretion, MALDI-MS, fermentation
Introduction and physiology of Gram-positive bacteria, and particularly of
sporulation and associated metabolism4; whereupon, infor-
Human growth hormone (hGH) is anionic, nonglycosylated mation concerning its secretion mechanism has been gath-
four helix-bundle protein known as somatotropin, having a ered as the genome sequence was resolved.5 Nevertheless,
molar mass of 22 kDa and 191 amino acid residues. It has the secretion of heterologous recombinant proteins from the
been used to treat hypopituitary dwarﬁsm, injuries, bone frac- bacilli might be inefﬁcient. On the basis of the growing
tures, bleeding ulcers, and burns.1 Recently, it appears to be of availability of information on genomics and proteomics of
considerable beneﬁt to girls with Turner’s syndrome, children B. subtilis, difﬁculties can now be systematically addressed
with chronic renal failure, and adults with growth hormone and overcome.6 For the secretion of a recombinant protein
deﬁciency or human immunodeﬁciency virus (HIV) syndrome.2 produced, either protease deﬁcient Bacillus cells7 or protease
Bacillus species, producers of several industrial enzymes, inhibitors are used. Westers et al.,8 in their review article,
are potential hosts for production of heterologous nonglyco- summarised the efforts employed to improve B. subtilis as a
sylated proteins of commercial interest. The advantages of host for protein secretion. Expression and secretion of non-
using the Gram positive bacteria, besides the ability to glycosylated proteins in the genus Bacillus require the assis-
secrete functional extracellular proteins directly into the bio- tance of the N-terminal signal-sequence of precursors.
reactor culture medium, are the lack of pathogenicity and the Brockmeier et al.9 and Fu et al.10 reported the use of various
absence of lipopolysaccharides (endotoxins) from the cell promoters and signal DNA sequences for recombinant pro-
wall.3 Amongst, Bacillus subtilis has become a model system tein production by B. subtilis.
for the study of many aspects of the biochemistry, genetics, Extracellular production of a recombinant foreign protein
from a B. subtilis host requires a neat design and engineering
Correspondence concerning this article should be addressed to T. H. of an expression and secretion system; wherein, the choice
Ozdamar at email@example.com. of the promoter and signal DNA sequence in combination
V 2009 American Institute of Chemical Engineers
76 Biotechnol. Prog., 2009, Vol. 25, No. 1
Table 1. Strains, Plasmids and Primers used in this Study
Name Description Reference or Source
Bacillus licheniformis Wild type carrying subC gene DSM 1969 (11)
B. subtilis nprÀaprÀ BGSC- 1A751
B. subtilis spoÀ BGSC- 1A179
Escherichia coli XL1Blue
pUC19::pre(subC)::hGH This work
pMK4::pre(subC)::hGH This work
Primers for pre(subC)::hGH
pre(subC) forward primer 50 _GCT CTA GAG CGC AAT CTC CTG TCA TTC G_30
Complimentary strand to hGH þ 50 _GGT ATA GTT GGG AAA GCA GAA GCG GAA TCG_30
Complimentary strand to pre(subC) þ 50 _GCT TCT GCT TTC CCA ACT ATA CCA CTA TCT C_30
hGH forward primer
hGH reverse primer 50 _GCG GAT CCG CAC TGG GGA GGG GTC AC_30
with the DNA vector, in particular, is important. For accession number A00501) from Homo sapiens and antibi-
recombinant protein production using Bacillus species, there otic resistance genes to ampicillin and tetracycline.
is no work in the literature reporting on the use of the pro- The primers used for the ampliﬁcation are given in Table
moter and signal sequence of the DNA encoding the indus- 1. Signal DNA sequence of subC was fused in front of the
trial enzyme serine alkaline protease (SAP). hGH gene using gene splicing by overlap extension
The idea in this work is based on the construction of a method.20 XbaI restriction site was incorporated to the for-
recombinant plasmid for the synthesis and secretion of rhGH ward primer of pre(subC) sequence, whereas BamHI restric-
that mimics the synthesis of SAP in bacilli. Thus, the tion site was incorporated to the reverse primer of hGH
hybrid-gene of two DNA fragments, i.e., signal (pre-) DNA gene. To verify the cloning, nucleotide sequencing analyses
sequence of B. licheniformis serine alkaline protease (SAP) were performed at Microsynth GmbH (Switzerland) using
gene (subC) and cDNA encoding hGH, were cloned into the designed primers.
pMK4 plasmid and expressed under the deg-promoter in a B.
subtilis host. Production of rhGH from B. subtilis and the
fermentation characteristics in a deﬁned medium were inves-
Culture maintenance and media for fermentation
tigated, using the designed hybrid-gene system.
For the bioprocess experiments, B. subtilis BGSC-1A751
(nprÀaprÀ) and B. subtilis BGSC-1A197 (spoÀ) stock cul-
Experimental Methods tures were maintained on agar slants that contained (g LÀ1):
peptone, 5; beef extract, 3; agar, 15; and initial pH ¼ 7.25.
Bacterial strains, plasmids, and growth media for The cells on the newly prepared slants were inoculated into
genetic manipulation the preculture medium for preparation of inocula that con-
The strains, plasmids, and primers used in this study are tained (g LÀ1): soytryptone, 15; peptone, 5; MnSO4.2H2O,
described in Table 1. Bacterial strains, plasmids, and growth 0.010; Na2HPO4, 0.25; CaCl2, 0.100 and grown at 37 C for
media were prepared using standard techniques.15 B. licheni- 6 h. The deﬁned reference production medium for batch-bio-
formis (DSM 1969), B. subtilis, and Escherichia coli XL1- reactor was as follows (g LÀ1): glucose, 6.0; (NH4)2HPO4,
Blue16 were maintained and grown on LB-agar that con- 4.7; KH2PO4, 2.0; 0.04 M Na2HPO4 and NaH2PO4; the ini-
tained (g LÀ1): tryptone, 10; NaCl, 5; yeast extract, 5; agar, tial pH ¼ 7.25.11,21 Chloramphenicol (7 lg/mL) was used in
15 and in LB broth (without agar) at 37 C. Ampicillin (100 all bioprocess experiments of plasmid-bearing B. subtilis.
lg/mL) was used for the plasmid maintenance in E. coli Complete EDTA-free protease inhibitor (Roche) was used to
strains; 7 lg/mL chloramphenicol was used for plasmid prevent proteolytic hydrolysis of the produced rhGH.
maintenance in the recombinant B. subtilis.
Laboratory-scale batch fermentations
Manipulation of DNA, PCR, cloning, and DNA sequencing Batch laboratory-scale fermentation experiments were con-
B. licheniformis chromosomal DNA was isolated as ducted in orbital shakers under agitation and heating rate
described by Posprech and Neumann.17 subC gene (GenBank control, using air-ﬁltered 500 mL Erlenmeyer-ﬂasks having
Acc. No. X03341)18,19 that encodes for extracellular serine 220 mL working volume capacities. Batch-bioreactor experi-
alkaline protease (SAP) enzyme of B. licheniformis was used ments were conducted in 1.0 L bioreactor systems (BBraun,
as the template for ampliﬁcation of signal (pre-) sequence. Germany) consisted of temperature, pH, foam, air inlet, and
HGH cDNA16 was ampliﬁed from E. coli host strain carry- stirring rate controls with 0.5 L working volume. Each
ing the plasmid pHGH107 (ATCC 31538; US patent no. experiment was conducted in two bioreactors operating in
4,342,832), featuring the growth hormone ORF (NCBI parallel, to check reproducibility.
Biotechnol. Prog., 2009, Vol. 25, No. 1 77
Analyses microliters of a 10 mg mLÀ1 sinapinic acid matrix dissolved
Cell concentrations based on dry weights were measured in 50% acetonitrile and 0.1% TFA solution, was mixed with
with a UV-vis spectrophotometer (Shimadzu UV-160A, To- 1 lL of $10 pmol lLÀ1 sample and 1 lL of this mixture
kyo, Japan) using a calibration curve obtained at 600 nm. was spotted on the target plate and air-dried by the ‘‘dried
Glucose consumption was followed by the glucose oxidation droplet’’ technique.28 Cytochrome c and Humatrope (stand-
method at 505 nm with UV-vis spectrophotometer.22 ard hGH) were used as molecular weight standards for pur-
Excreted amino acid concentrations were measured with an poses of mass correction. Spectra were generated from the
amino acid analysis system (Waters HPLC, Milford, MA), sum of 100–200 laser pulses and mass determinations were
using the Pico Tag method.23 Excreted organic acid concen- made by ﬁnding the peak centroid of a smoothed signal (by
trations were measured with an HPLC (Waters, HPLC, Alli- Savitzky-Golay algorithm) after background subtraction.29
ance 2695).24 HGH concentrations were measured using a
high-performance capillary electrophoresis (Waters HPCE, Results
Quanta 4000 E, Milford, MA). The samples were analyzed
at 12 kV and 15 C with a positive power supply using 60 cm Construction of the plasmid pMK4::pre(subC)::hGH
Â 75 lm silica capillary using modiﬁed 100 mM borate B.licheniformis (DSM 1969) chromosomal DNA and
buffer (pH ¼ 10) including zwitterions (Z1-Methyl, Waters) pHGH107 plasmid, containing the hGH cDNA, were isolated
as the separation buffer. Proteins were detected by UV ab- to be used as templates in PCR reactions. The two target
sorbance at 214 nm, as mentioned elsewhere.23,25 Humatrope genes, pre(subC) of subC gene (360 bp) from B. lichenifor-
(Eli Lilly, France) was used as the standard. The Dynamic mis chromosomal DNA and mature peptide sequence of
method26 was applied to ﬁnd the oxygen uptake rate (OUR) hGH (639 bp) from pHGH107 plasmid, were ampliﬁed by
and oxygen transfer coefﬁcient (KLa) values. PCRs (Figure 1). The primers (Table 1) used at the ends to
The physiological data for each operation were from at be joined were designed as complementary to one another
least two independent experiments, and the average values by including nucleotides at their 50 ends that are complemen-
were given. tary to the 30 portion of the other primer. The two PCR prod-
ucts containing the overlapping fragments at the ends to be
Ultraﬁltration and puriﬁcation joined were puriﬁed and, by the third PCR reaction using the
Concentration and desalting of the production medium external primers carrying XbaI and BamHI restriction sites,
was achieved by ultraﬁltration under nitrogen gas (55 psi, extension of the overlap by DNA polymerase has yielded the
3.8 bar) at 4 C using Amicon 400 mL stirred pressure cells hybrid-gene product, i.e., pre(subC)::hGH (999 bp), where
(Millipore, Bedford, MA) with regenerated cellulose ultraﬁl- pre(subC) DNA sequence, was fused in front of the hGH
tration membranes having MWCO of 10 kDa (Millipore, sequence (Figure 1). The hybrid-gene pre(subC)::hGH was
Bedford, MA). Puriﬁcation of rhGH was achieved by then cloned into the XbaI and BamHI sites of pUC19 E. coli
aptamer-based afﬁnity chromatography. Concentrated sam- plasmid, and transformed into E. coli XLI-Blue cells by
ples were mixed with hGH speciﬁc aptamer which was im- CaCl2 method. Thereafter, pre(subC)::hGH was sub-cloned
mobilized onto microparticles and hGH-aptamer binding was to pMK4 SalI and BamHI sites and expressed in the hosts B.
carried out at 25 C for 30 min, which has been developed subtilis BGSC-1A751 (nprÀaprÀ) and B. subtilis BGSC-
and is being studied in our research group. 1A197 (spoÀ).
SDS-PAGE, Western Blotting, and N-terminal SDS-PAGE, Western Blot, N-terminal, and mass
sequence analysis spectrometry analyses
Sodium dodecyl sulfate-polyacrylamide gel electrophoresis RhGH production potential of the recombinant cells, B. subti-
(SDS-PAGE) was performed as described by Laemmli27 by lis BGSC-1A751 (nprÀaprÀ) carrying pMK4::pre(subC)::hGH
using 4% stacking and 12% separating polyacrylamide gel, run
on a Mini Protean II DUAL SLAB cell (Bio-Rad) according
to the manufacturer’s instructions and silver stained. For West-
ern blot analysis, polyclonal rabbit anti-human growth hor-
mone (BioMeda, USA) was used as the primary antibody and
horseradish peroxidase labeled goat-anti rabbit IgG (HþL)
(BioMeda) was used as the secondary antibody. For the N-ter-
minal analysis, rhGH was electrophoresed as described above
and transferred onto a polyvinylidene diﬂuoride membrane
(Millipore, USA). After being stained with Coomassie blue,
the rhGH band was excised, and automated Edman degrada-
tion was performed by PROCISE 494 gas-phase/liquid-pulse
sequencer (Applied Biosystems, Foster City, CA).
MALDI-ToF mass spectrometry analysis
The molecular weight of rhGH was determined by the use
of a MALDI-LR (Waters-Micromass, UK) instrument. Spec- Figure 1. Agarose gel electrophoresis view for PCR ampliﬁca-
tra were generated using a pulsed nitrogen gas laser tion of hGH, pre(subC), and pre(subC)::hGH.
(337 nm) in positive linear mode with a low mass gate of M, low range marker (Fermantas); Lane 1, hGH; Lane 2, pre
1,000 Da.25 The accelerating voltage was 15 kV. Three (subC)::hGH; and Lane 3, pre(subC).
78 Biotechnol. Prog., 2009, Vol. 25, No. 1
Figure 2. Western Blott analysis results of BGSC-1A751
(npr2, apr2) carrying pre(subC)::hGH and BGSC-
1A197 (spo2) carrying pre(subC)::hGH.
Lane 1, commercial (standard) hGH; Lane 2, hGH produced by
r-B. ‘subtilis BGSC-1A751 (nprÀ, aprÀ) carrying pMK4::pre
(subC)::hGH; Lane 3, hGH produced by r-B. subtilis BGSC-
1A197 (spoÀ) carrying pMK4::pre(subC)::hGH; and Lane 4,
marker (Sigma M 0671). Figure 3. SDS-PAGE analysis of rhGH, produced by r-B.subti-
lis BGSC-1A751 (npr2, apr2) carrying pMK4::
M, protein marker (Fermentas); Lane 1, product mixture of
and B. subtilis BGSC-1A197 (spoÀ) carrying pMK4::pre r-B.subtilis containing rhGH; Lane 2, 1st rhGH separation with
hGH speciﬁc aptamer; Lane 3, 2nd rhGH separation with hGH
(subC)::hGH were determined on a glucose (CG ¼ 6 g LÀ1) speciﬁc aptamer; and Lane 4, standard hGH.
based deﬁned medium. The supernatant obtained by centrifuga-
tion at t ¼ 27 h of the fermentation was partially puriﬁed by
dead-end ultraﬁltration and nearly 10-fold concentration was the ﬁrst six amino acid residues of the putative rhGH prod-
achieved. Western blot analysis showed that (Figure 2), the mo- uct were Phe-Pro-Thr-Ile-Pro-Leu, identical to the true hGH
lecular mass of rhGH produced by B. subtilis BGSC-1A751 car- sequence. In support of this, nucleotide sequencing results
rying pMK4::pre(subC)::hGH and B. subtilis BGSC-1A197 were also 100% matching.
carrying pMK4::pre(subC)::hGH was 22 kDa being the same as
the standard hGH (Humatrope, Eli Lilly, France).
For further characterization, rhGH was puriﬁed, from 10-
fold concentrated and partially puriﬁed fermentation broth Host selection and effect of glucose concentration on
(Figure 3, Lane 1), by aptamer-based afﬁnity chromatogra- rhGH fermentation
phy, which has been developed and is being studied in our Effects of initial glucose concentration on the recombinant
research group. Concentrated samples were mixed with hGH cells were investigated in laboratory-scale experiments by B.
speciﬁc aptamer and hGH-aptamer binding was carried out subtilis BGSC-1A751 and B. subtilis BGSC-1A197 carrying
at 25 C for 30 min (Figure 3, Lane 2). To obtain higher pu- pMK4::pre(subC)::hGH, at the initial concentrations of
riﬁcation, the aptamer-afﬁnity separation step was applied CGo ¼ 6.0, 8.0, 10.0, and 15.0 g LÀ1. The variations in glu-
second time and after the elution step, rhGH was found to cose, cell and rhGH concentrations with the cultivation time
be separated from the fermentation broth with 99.8% purity by B. subtilis BGSC-1A751 carrying pMK4::pre(subC)::hGH
and 41% overall yield (Figure 3, Lane 3), the molecular are presented in Figures 5a–c, respectively. The cell concen-
mass of puriﬁed rhGH was determined by MALDI-ToF mass tration was not affected from the initial glucose concentra-
spectrometry (MS), to verify the structure of the secreted tion at t ¼ 0–6 h. Because of the addition of the protease
recombinant hormone. A commercial preparation (standard) inhibitor at t ¼ 6 h, an interruption in the cell growth was
of rhGH was analyzed ﬁrst, and showed a spectral peak at observed until t ¼ 15 h. However, after t ¼ 15 h the second
m/z 22,126 (Figure 4a). The stated molecular mass of the cell growth phase was started with the initiation of rhGH
standard hormone is 22,125 Da. Thus, the detected ion was synthesis (Figure 5c). The highest cell concentration was
[MþH]þ, with the exact molecular mass of m ¼ 22,126 Da, obtained at CGo ¼ 15 g LÀ1 at t ¼ 36 h as CX ¼ 2.8 g LÀ1
where the charge on the ion is z ¼ þ1. The native length of (Figure 5a). In the ﬁrst 15 h, the glucose consumption rates
the protein was obtained as the peak at m/z 22,133 (Figure were close to each other; however at t [ 15 h, parallel to
4b), corresponding to the [MþH]þ ion of rhGH detected the cell growth proﬁles, with the increase in cell growth rate,
with just a 0.03% error, which is a reasonable error at high the glucose consumption rate increased being the highest at
molecular weights in MALDI-ToF MS analysis. This result CGo ¼ 15 g LÀ1. The highest rhGH was produced at CGo ¼
indicates that rhGH molecule was synthesised by the 8 g LÀ1 at t ¼ 36 h as CrhGH ¼ 30 mg LÀ1. On the other
recombinant construct pMK4::pre(subC)::hGH, and then hand, when the protease inhibitor was not used rhGH was
secreted into the fermentation medium properly. not detected in the fermentation broths of B. subtilis BGSC-
The amino acid sequence of the signal peptide fused in 1A751 and B. subtilis BGSC-1A197 carrying pMK4::pre
front of the rhGH was: ‘‘MMRKKSFWLGMLTA (subC)::hGH.
FMLVFTMAFSDSASA;’’ and, the N-terminal and mass A parallel set of experiments were conducted by B. subti-
spectrometry analyses indicated that the SAP signal peptide lis BGSC-1A197 carrying pMK4::pre(subC)::hGH. Similar
was properly processed by the B. subtilis signal peptidase to B. subtilis BGSC-1A751 results, the highest rhGH produc-
(Spase I), because the results of N-terminal sequencing of tion was obtained at CGo ¼ 8 g LÀ1 but with a lower rhGH
Biotechnol. Prog., 2009, Vol. 25, No. 1 79
Figure 4. MALDI-ToF MS analysis.
(a) Standard hGH (b) B. subtilis produced and puriﬁed rhGH.
value (CrhGH ¼ 26 mg LÀ1). On the basis of the results, B. by B. subtilis carrying merely pMK4, as can be seen in Fig-
subtilis BGSC-1A751 was selected as the host. ure 6.
In the process by B. subtilis carrying pMK4, the glucose
consumption was higher until t ¼ 18 h of the fermentation,
but after t ¼ 18 h, it was almost zero; where the highest cell
Inﬂuence of hGH gene on the physiology of r-Bacillus concentration and the overall cell yield on substrate (YX/S)
subtilis were obtained as CX ¼ 1.6 g LÀ1 (t ¼ 12 h) and YX/S ¼
To determine the inﬂuence of hGH gene on the physiol- 0.23 g gÀ1, respectively. Contrarily, by B. subtilis carrying
ogy of the bacilli, bioreactor experiments were performed by pMK4::pre(subC)::hGH, the glucose consumption was
B. subtilis BGSC-1A751 carrying merely pMK4, and B. sub- increased after t ¼ 18 h and the highest cell concentration
tilis BGSC-1A751 carrying pMK4::pre(subC)::hGH, at T ¼ was obtained as CX ¼ 2.0 g LÀ1 (t ¼ 24 h); and the overall
37 C, pH0 ¼ 7.25, CGo ¼ 8 g LÀ1, air inlet rate of 0.5 vvm cell yield on substrate was YX/S ¼ 0.25 g gÀ1.
and agitation rate of 800 minÀ1. The concentrations of the As expected, rhGH production was achieved only by B.
glucose, cell, extracellular rhGH, the by-products amino and subtilis carrying pMK4::pre(subC)::hGH. RhGH synthesis
organic acids, together with the oxygen-uptake (OUR) rates, started at t ¼ 18 h of the batch-bioprocess and increased
oxygen-transfer coefﬁcients (KLa), and yield coefﬁcients with the cultivation time reaching the value CrhGH ¼ 70 mg
were determined throughout the fermentations. LÀ1 at t ¼ 32 h. The highest product yield on substrate was
The variations in dissolved oxygen concentration (CO) and obtained at 24 t 32 h as YrhGH/S ¼ 0.65 g gÀ1, while
pH with the cultivation time are presented in Figure 6; and the the overall rhGH yield on substrate was YrhGH/S ¼ 9 g kgÀ1.
variations in glucose, cell, and rhGH concentrations are pre- The excreted amino acids that were detected at considerable
sented in Figure 7. The loci of the CO vs. t proﬁles obtained concentrations in both fermentations are leucine, isoleucine,
in the two fermentation processes were similar until t ¼ 18 h, and phenylalanine. The highest concentrations of leucine, iso-
where considerable decrease in dissolved oxygen concentration leucine, and phenylalanine by B. subtilis carrying pMK4::pre
was observed at t ¼ 0–4 h. However, in the process by B. sub- (subC)::hGH were 0.191, 0.096, and 0.132 g LÀ1; whereas by
tilis carrying pMK4::pre(subC)::hGH, a considerable decrease B. subtilis carrying pMK4 were as 0.214, 0.107, and 0.281 g
in CO between t ¼ 18 h and t ¼ 22 h was detected due to LÀ1, respectively. Nevertheless, alanine, arginine, asparagine,
rhGH synthesis that induced the cell growth. By the termina- aspartic acid, glutamic acid, glycine, histidine, methionine, ly-
tion of the cell formation, a breakthrough in dissolved oxygen sine, valine, treonine, and tryptophan were not detected in the
concentration at t ¼ 22 h is observed. broths of the two fermentation processes. Thus, the total
The pH in both fermentation media decreased until t ¼ excreted amino acid concentrations were higher in the fermen-
18 h. After t ¼ 18h, pH continued to decrease by depicting tation by B. subtilis carrying merely pMK4.
a characteristic curve by B. subtilis carrying pMK4::pre Variations in excreted organic acid concentration produced
(subC)::hGH; contrarily, pH was increased in the bioreactor by B. subtilis carrying pMK4::pre(subC)::hGH and B.
80 Biotechnol. Prog., 2009, Vol. 25, No. 1
Figure 5. (a) Variation in cell concentration with the cultivation time for B. subtilis BGSC-1A751 (npr2apr2) carrying pMK4::pre
(subC)::hGH with the initial glucose concentration. CGo(g L21): (^) 6.0; () 8.0; (~) 10.0; (*) 15.0. (b) Variation in glu-
cose concentration with the cultivation time for B. subtilis BGSC-1A751 (npr2apr2) carrying pMK4::pre(subC)::hGH with
the initial glucose concentration. CGo(g L21): (^) 6.0; () 8.0; (~) 10.0; (*) 15.0. (c) Variation in rhGH concentration
with the cultivation time for B. subtilis BGSC-1A751 (npr2apr2) carrying pMK4::pre(subC)::hGH with the initial glucose
concentration. CGo(g L21): (^) 6.0; () 8.0; (~) 10.0; (*) 15.0 as in 5c.
subtilis carrying pMK4 are presented in Figures 8a,b, respec- Fermentation characteristics
tively. Oxaloacetic acid, which is known to be produced in The variations in KLa and the oxygen uptake rate (OUR)
cell regeneration, was not excreted in both fermentations. In are presented in Table 2. Considering the characteristic cell
the fermentation by B. subtilis carrying pMK4::pre
and rhGH concentration proﬁles of B. subtilis carrying
(subC)::hGH, the main extracellular by-products were suc-
pMK4::pre(subC)::hGH, the bioprocess was divided into ﬁve
cinic, gluconic and formic acid (Figure 8a). Lactic, oxalic,
periods. 0 t 4 h is the cell ﬁrst-growth-phase; 4 t
citric acids are the organic acids having lower concentra-
12 h is the growth-interruption-phase; 12 t 16 h is the
tions, i.e., ca. 0.1 g LÀ1; moreover, pyruvic, a-ketoglutaric,
and acetic acids were detected at a concentration of ca. lag-phase where rhGH synthesis starts; 16 t 24 h is the
0.01 g LÀ1. Indeed, the organic acid proﬁles obtained by B. second-cell-growth-phase where rhGH synthesis increases;
subtilis carrying pMK4::pre(subC)::hGH are different than and 24 t 32 h is the end of the growth-phase where
that of B. subtilis carrying pMK4; where in the latter, the rhGH synthesis was the highest.
main excreted by-products were malic and gluconic acids In both fermentation processes, the oxygen transfer coefﬁ-
(Figure 8b), and the concentration of the other organic acids cient, KLa, increased with the increase in the cultivation
were lower than 0.01 g LÀ1. Thus, the amount of total or- time, and then decreased. At 0 t 4 h of the bioprocess,
ganic acids excreted by B. subtilis carrying pMK4::pre KLa and oxygen uptake rate of B. subtilis carrying pMK4
(subC)::hGH were higher. was higher than that of B. subtilis carrying hGH gene.
Biotechnol. Prog., 2009, Vol. 25, No. 1 81
Figure 6. Variations in dissolved oxygen concentration and pH
with the cultivation time by B.subtilis BGSC-1A751 Figure 7. Variations in glucose, cell, and hGH concentrations
(npr2apr2) carrying pMK4::pre(subC)::hGH (r- with the cultivation time by B. subtilis BGSC-1A751
pMK4) and BGSC-1A751 (npr2apr2) carrying (npr2apr2) carrying pMK4::pre(subC)::hGH and
pMK4. Co: continuous lines, pH: dashed lines. BGSC-1A751 (npr2apr2) carrying pMK4. Glucose
concentration: (--n--) pMK4; () pMK4::pre
(subC)::hGH, Cell concentration: (--l--) pMK4; (*)
Throughout the bioprocess, the highest KLa value was pMK4::pre(subC)::hGH, rhGH concentration: (~)
obtained by B. subtilis carrying merely pMK4 as KLa¼ pMK4::pre(subC)::hGH.
0.028 sÀ1 at 0 t 4 h; however, due to low oxygen con-
centrations within t ¼ 4–16 h in the production medium, the
dynamic method could not be applied. Related with B. subti- spectrometry analyses indicate that the signal peptidase has
lis carrying pMK4::pre(subC)::hGH, the highest OUR values cut at the site within Spase I group. Thus, the system
were obtained in the ﬁrst growth- (0 t 4 h) and second- designed functioned with its intended purpose effectively in
cell-growth- phases (16 t 24 h) (Table 2). expression and cleavage of the recombinant product.
The other peak in Figure 4b at m/z 25,854 is possibly an
unspeciﬁcally bound impurity protein to the hGH-ligand during
Discussion and Conclusions puriﬁcation. From the facts that MALDI-ToF MS can not be
used in quantitation of proteins because protein detection
An expression system producing therapeutic protein depends on ionisation efﬁciency and that even femto-mole
human growth hormone that conceptually mimics the extrac- amounts can be detected, and from SDS-PAGE (Figure 3,
ellular serine alkaline protease synthesis in the genus Bacil- Lane 3) analysis where a 25.8 kDa band was not detected, it
lus was designed and implemented. For the extracellular was concluded that the impurity was in negligible amounts.
production of human growth hormone by B. subtilis, a Furthermore, the peaks detected at m/z 20.4 kDa in Figures
recombinant plasmid carrying the hybrid-gene of two DNA 4a,b, are possibly the cleaved 20 kDa forms of hGH.32
fragments, i.e., signal (pre-) DNA sequence of a Bacillus The constructed expression system produces extracellular
licheniformis extracellular SAP enzyme gene (subC) and the rhGH from B. subtilis starting from the beginning of the fer-
DNA encoding hGH, was constructed and transferred into mentation process parallel to the cell growth, giving a break-
two host Bacillus strains, namely B. subtilis BGSC-1A751 through at t ¼ 12 h. RhGH concentration was the highest at
(nprÀaprÀ) and B. subtilis BGSC-1A197 (spoÀ). These t ¼ 32 h as CrhGH ¼ 70 mg LÀ1 and overall speciﬁc-product
strains were selected for their deﬁciencies in two protease yield on substrate was YrhGH/S ¼ 9 g kgÀ1, in the deﬁned
genes and sporulation gene, respectively. RhGH, expressed medium with sole carbon source glucose. Nakayama et al.,33
by the hybrid-gene pre(subC)::hGH cloned into pMK4 in reported rhGH secretion level of 40 mg LÀ1 which is 1.75-
both hosts, was secreted. fold lower than that obtained in this study. Kajino et al.,34
The approach developed is expected to be applicable to modiﬁed the ‘‘middle wall protein (MWP) signal peptide’’ of
the design of expression systems for heterologous protein B. brevis and constructed a hGH expression system and
productions from Bacillus. As the rhGH concentration reported rhGH secretion from B. brevis with an overall spe-
obtained from the protease-deﬁcient host B. subtilis BGSC- ciﬁc-product yield on glucoseþpolypeptone YrhGH/S ¼ 4 g
1A751 was higher than that of the host B. subtilis BGSC- kgÀ1 which is 2.25-fold lower than the value reported in this
1A197, the ﬁrst was selected as the host owing to two gene work. Related with a different expression system for human
deletions targeting the decrease in protease activities. interleukin-3, Westers et al.,35 reported 0.1 g LÀ1 recombi-
Secreted proteins are generally synthesised as precursors nant human interleukin-3 secretion by an eight-protease-deﬁ-
with a cleavable signal peptide, and then the signal peptide cient B. subtilis, using a semi-deﬁned enriched medium;
is removed by signal peptidases, where preprotein processing where the overall speciﬁc-product yield on substrate was
by signal peptidases are essential for bacterial growth and vi- lower then the YrhGH/S value reported in this work. There-
ability.30,31 The native length of rhGH was detected in SDS- fore, we conclude that the expression system designed,
PAGE (Figure 2, Lane 2), Western blot (Figure 3, Lane 3) which is based on the idea of using the ribosomal binding
and MALDI-ToF MS analysis as the peak at m/z 22,133 site- promoter- and the signal peptide of serine alkaline pro-
(Figure 4b) with just a 0.03% error, which is a reasonable tease enzyme gene subC, is successful for the extracellular
error at high molecular weights. The N-terminal and mass recombinant protein production.
82 Biotechnol. Prog., 2009, Vol. 25, No. 1
Figure 8. (a) Variation in organic acid concentrations with the cultivation time by B. subtilis BGSC-1A751 (npr2apr2) carrying
pMK4::pre(subC)::hGH. (b) Variation in organic acid concentrations with the cultivation time by B.subtilis BGSC-1A751
(npr2apr2) carrying pMK4.
Table 2. Variation in Oxygen Transfer Characteristics with the Cultivation Time
Microorganism Period KLa (sÀ1) (mol mÀ3 sÀ1)
BGSC-1A751 (nprÀaprÀ) First growth phase, 0 t 4 h 0.017 3.5
carrying pMK4::pre(subC)::hGH Growth-interruption-phase, 4 t 12 h 0.018 3.0
Lag-phase and rhGH 0.014 2.4
synthesis phase, 12 t 16 h
Second cell growth and rhGH 0.015 3.5
synthesis phase, 16 t 24 h
End of the growth and rhGH 0.010 0.4
synthesis phase, 24 t 32 h
BGSC-1A751 (npr apr ) 0t4 h 0.028 4.6
carrying pMK4 4 t 12 h – –
12 t 16 h – –
16 t 24 h 0.013 0.4
24 t 32 h 0.010 0.3
The transcription for rhGH synthesis by B. subtilis BGSC- (subC)::hGH, on glucose as sole carbon source. The results
1A751 (nprÀaprÀ) carrying the hybrid-gene pre reveal that the expression of rhGH inﬂuences the physiology
(subC)::hGH is under the control of degQ promoter; there- of the r-Bacillus cells, as expected. The cell concentration
fore, the synthesis and secretion pattern of rhGH mimics the proﬁle of B. subtilis carrying pMK4::pre(subC)::hGH shows
synthesis and secretion of SAP enzyme in B. lichenifor- a perturbed biphasic variation because of the introduction of
mis.11,21,36 Because the ribosomal binding site, promoter, a protease inhibitor at t ¼ 4 h, where a drastic decrease in
and signal peptide of the SAP gene (subC) were used in the the growth rate occurs until t ¼ 16 h, that proceeds with an
constructed expression system for the synthesis and secretion increase in the growth rate until ca. t ¼ 24 h.
of rhGH, a similar rhGH concentration proﬁle to that of RhGH synthesis and secretion proceeded until t ¼ 32 h,
the SAP productions from B. licheniformis carrying giving the highest concentration as CrhGH ¼ 70 mg LÀ1. As
pHV1431::subC21 and B. subtilis carrying pHV1431::subC11 expected, there was no rhGH production by the microorgan-
was obtained. The slight difference observed in the concen- ism that does not carry hGH gene.
tration proﬁles was likely due to an interruption caused by Due to the introduction of new biochemical reactions into
the addition of protease inhibitors. Therefore, we conclude the intracellular reaction network producing the heterologous
that the expression and secretion system constructed for extracellular protein, the fermentation and oxygen transfer
rhGH production from the genus Bacillus is dependent on characteristics and by-product distributions of B. subtilis car-
the bioreactor operation conditions (which is being studied), rying pMK4::pre(subC)::hGH were different than that of the
similar to that of SAP production by Bacillus species.37 B. subtilis carrying merely pMK4. Moreover, higher concen-
To investigate the inﬂuence and perturbation effect of trations of organic acids detected in the broth of B. subtilis
hGH gene on the physiology of r-B. subtilis, comparative carrying pMK4::pre(subC)::hGH, and higher concentrations
bioreactor experiments were performed by B. subtilis carry- of amino acids detected in the broth of B. subtilis carrying
ing merely pMK4 and B. subtilis carrying pMK4::pre pMK4, reveal the impact of the structure of the plasmids on
Biotechnol. Prog., 2009, Vol. 25, No. 1 83
the synthesis capacity of the host throughout the Moestl D, Nakai S, Noback M, Noone D, OReilly M, Ogawa
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This work was supported by the Scientiﬁc and Technical secretory production of intact, biologically active staphylo-
Research Council of Turkey (TUBITAK) through the projects kinase from Bacillus subtilis. Biotechnol Bioeng. 1999;62:87–
104M012 and 107M420. Ankara University Biotechnology 96.
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spectrometer. Humatrope was supplied kindly by Pharmacist tory for pharmaceutical proteins: a biotechnological approach to
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