Bacterial Conjugation (Genetic recombination in Bacteria)
1. Bacterial Conjugation
o Conjugation is merely the fusion of two
compatible bacterial cells.
o Bringing two genotypes together and allowing
them to conjugate is the equivalent of making
a cross in eukaryotes.
o Our discussion of conjugation will center on
the gut bacterium Escherichia coli (E.
coli). Conjugation and gene transfer in E.
coli are driven by a
circular DNA plasmid called the fertility
factor or sex factor (F), which is found in some
but not all cells.
o Hence to understand how to make a cross in E.
coli, we have to understand the properties of F.
2. Joshua Lederberg and Edward Tatum, who studied
two strains of Escherichia coli with different
nutritional requirements.
Strain A would grow on a minimal medium only if the
medium were supplemented with methionine and
biotin; strain B would grow on a minimal medium
only if it were supplemented with threonine, leucine,
and thiamine.
Thus, we can designate strain A
as met− bio− thr+ leu+ thi+ and strain B
as met+ bio+ thr− leu− thi−.
Here, strains A and B are mixed together, and some of
the progeny are now wild type, having regained the
ability to grow without added nutrients.
Discoveryof conjugation
3. Properties of the F Plasmid
Cells carrying the F plasmid are designated F+, and those lacking it are F−. The F plasmid
contains approximately 100 genes, which give the plasmid several important properties:
o The F plasmid can replicate its own DNA, allowing the plasmid to be maintained in a
dividing cell population
o Cells carrying the F plasmid promote the synthesis of pili (singular, pilus) on the
bacterial cell surface.
o Pili are minute proteinaceous tubules that allow the F+ cells to attach to other cells and
maintain contact with them; that is, to conjugate.
o F+ and F− cells can conjugate. When conjugation occurs, the F+ cells can act as F
donors. The F plasmid DNA replicates and the newly synthesized copy of the circular F
molecule is transferred to the F− recipient.
4. o However, a copy of F always remains behind in the donor cell. The
recipient cell becomes converted into F+, because it now contains a circular
F genome. The transfer of the F plasmid from F+ to F− is rapid, so the F
plasmid can spread like wildfire throughout a population from strain to
strain.
o F+ cells are usually inhibited from making contact with other F+ cells;
therefore the F plasmid is not transferred from F+ to F+.
Donor cells typically act as donors because they have a chunk of DNA
called the fertility factor (or F factor). This chunk of DNA codes for the
proteins that make up the sex pilus. It also contains a special site where
DNA transfer during conjugation begins.
If the F factor is transferred during conjugation, the receiving cell turns
into an F^+ + start superscript, plus, end superscript donor that can make
its own pilus and transfer DNA to other cells
6. Bacterial Conjugation Steps
In order to transfer the F-plasmid, a donor cell and a recipient cell must first establish contact. At this
point, when the cells establish contact, the F-plasmid in the donor cell is a double-stranded
DNA molecule that forms a circular structure.
The following steps allow the transfer of the F-plasmid from one bacterial cell to another:
Step 1
The F+ (donor) cell produces the pilus, which is a structure that projects out of the cell and begins
contact with an F– (recipient) cell.
Step 2
The pilus enables direct contact between the donor and the recipient cells.
Step 3
Because the F-plasmid consists of a double-stranded DNA molecule forming a circular structure, i.e., it
is attached on both ends, an enzyme (relaxase, or relaxosome when it forms a complex with other
proteins) nicks one of the two DNA strands of the F-plasmid and this strand (also called T-strand) is
transferred to the recipient cell.
7. Step 4
In the last step, the donor cell and the recipient cell, both containing single-
stranded DNA, replicate this DNA and thus end up forming a double-stranded F-
plasmid identical to the original F-plasmid.
Given that the F-plasmid contains information to synthesize pili and other proteins
,the old recipient cell is now a donor cell with the F-plasmid and the ability to form
pili, just as the original donor cell was.
Now both cells are donors or F+.
10. An important breakthrough came when Luca Cavalli-Sforza discovered a derivative of an F+ strain.
On crossing with F− strains this new strain produced 1000 times as many recombinants for genetic markers as
did a normal F+ strain.
Cavalli-Sforza designated this derivative an Hfr strain to indicate a high frequency of recombination.
In Hfr × F− crosses, virtually none of the F− parents were converted into F+ or into Hfr. This result is in
contrast with F+ × F− crosses, where infectious transfer of F results in a large proportion of the F− parents
being converted into F+.
It became apparent that an Hfr strain results from the integration of the F factor into the chromosome,
Hfr strains
