Master of science #msc level

1. OPERON
*A unit made up of linked genes which are regulate other genes
responsible for protein synthesis.
*These linked genes that produces a single messenger RNA molecule in
transcription that consists of structural genes and regulating elements
such as an operator and promoter.
*A regulatory gene codes for regulatory protein.
• An operator is the region of DNA of the operon that is binding site for
the regulatory protein.
• Promoter is a region of DNA where transcription of a gene is initiated.
* It contain repressor and inducer also.

2. Arabinose operon
 The arabinose operon also called ara or araBAD operon.it is required for the
breakdown of the five carbon sugar L- arabinose in Escherichia coli.
 It is a complex regulatory system, it was first studied b y Ellis englesberg.
 He proposed arabinose operon in 1965. based on his work with the e coli .
 It is an inducible operon, because it is on when arabinose enters to the
cell.
 Arabinose operon can be regulated both positively and negatively .
 What is arabinose?
 Arabinose is a pentose sugar. It found in cell wall of plants. It can serve
as a carbon source for metabolism. It ie released in the human intestine
after vegitebles are eaten. It is not absorbed by the intestine and hence
provides a source of carbon for bacterial inhibitants of the intestine such
as E coli .

3.  The arabinose enters to cell membrane of e coli.it enters as L-arabinose.
 In the presence of araA it converted into L- ribulose.
 Then it converted to L- ribulose phosphate in the presence of
araB.(phosphorylation).
 L- ribulose phosphate converted into D- xylulose phosphate. then it converted
to simple sugar via HMP pathway.

6. Structure of arabinose operon
 For the successful utilization of arabinose e coli needs 2operons and one
gene.
1)ara BAD
2)araFGH
Gene- araE
araBAD
 araBAD is structural gene.They are araA,araB,araD.
 These genes encode three enzyme involved in the catabolism of arabinose.
 Ara O2 is the promoter.but it is distantly placed,
 araC is a regulatory protein,

7. araC in the presence of arabinose shows positive regulation.in the absence shows
negative regulation.
If c protein binds with l1 and l2 transcription gets activate.if the c protein binds with
02 transcription gets repressed.if c protein bind with O1 it act as repressor of
araC.ie, it repeats its own transcription.it is called autpregulation.
 Pc is the promter of araC.
 araO1 is is operator of araC.
 As- is activation site.
 I1 and I2 –binding site of C protein.
 araA encoded by isomerase.
 araB encoded by kinase.
 araD encoded by epimerase.

8. araBAD

9.  Ara BADis two directional. arabinose operon is the best evidence for the
transcription which takes place in two direction. Because ara C and structural
genes are transcribes in opposite directions.
POSITIVE REGULATION
When arabinose is present it binds to araC proteun and changes araC
confirmation.
An arabinose-ara C dimer complex binds prefentially to aral and not to araO2
which causes opening of the loop,
This allows RNA polymerase to bind to P BAD.
If glucose levels are low cAMP-CAP complex binds to Pc and activates the
transcription.

10. POSITIVE REGULATION
araCdimer

11.  NEGATIVE REGULATION
 When arabinose is absent the araC protein acts as a negative regulator,
 Ara c acts as a dimer and causes the DNA loop.
 Looping brings the aral and ara O2 sites in proximity to one another.
 One araC monomer binds to aral and second monomer binds to araO2.
 Binding of araC prevents RNA pol from binding to the P BAD promoter.
 Then transcription not occur.

12. NEGATIVE REGULATION

13.  If mutation happens,
 A mutation in the ara A will cause the bacterial cell to become arabinose
negative, bacteria no longer uses arabinose as carbon source.
 Ara B gene results the same as mutation in the araA.
 araD gene mutation results in cell death.
 Mutation of araC gene cause promoters pBAD and pC to become inactive and
permenantly repressed.