The arabinose operon ….

1. The arabinoseoperon…. Dual positive and negative control Presented by: Shilpisaikia Roll no: 04

2. The operon model • The operon model was first proposed by Jacob and Monod in 1961 to explain the regulation of genes encoding the enzymes required for lactose utilization in E.coli. • The operon consist of the following components: regulator gene, promoter gene, operator gene, structural gene, repressor and inducer.

3. The arabinoseoperon: • Complex regulatory system • First studied by Ellis Englesberg • Arabinoseoperon can be regulated both positively and negatively • Exhibits catabolite repression

4. Structure of arabinoseoperon: • Thearabinose(ara) operon of E.coli contains three structural genes (araB, araA, and araD) that encode the three enzymes involved in the catabolism of arabinose : • Arabinoseisomerase - encoded by araA - coverts arabinose to ribulose • Ribulokinase - encoded by araB -- phosphorylatesribulose

5. Ribulose-5-phosphate epimerase - encoded by araD -- converts ribulose-5-phosphate to xylulose-5-phosphate which can then be metabolized via the pentose phosphate pathway. • The three structural genes are arranged in an operon that is regulated by the araCgene product (regulatory protein). • The araCprotein regulates its own synthesis by repressing transcription of its gene. This phenomenon is called autoregulation.

7. The araoperon includes two operators (ara01 and ara02), another binding site for the araCregulatory protein called araI (I for inducer), and a promoter adjacent to araI. • A CAP binding site is adjacent to the araoperon promoter, and transcription is modulated by CAP-cAMP. • araCregulates its own synthesis, binding at ara01 and repressing transcription of the araC gene. • Second, it acts as both a positive and a negative regulator of the araBAD genes, binding to ara02 and araI.

9. Regulation of arabinoseoperon: • When araCprotein is depleted, the araC gene is transcribed from its own promoter. • When arabinose levels are low and glucose levels high, araCprotein binds to both araI and ara02 and brings these sites together to form a DNA loop. The operon is repressed in this state. araCprotein also binds to ara01, repressing further synthesis of araC. • When arabinose is present and glucose concentration is low, araCprotein binds arabinose and changes conformation to become an activator. The DNA loop is opened, and the araCprotein acts in with CAP-cAMPto facilitate transcription.

10. Mutations in the arabinoseoperon • A mutation in the araA gene will cause the bacterial cell to become arabinose negative. Bacteria no longer uses arabinose as carbon source. • araBgene mutation results the same. • araD gene mutation results in cell death. • Mutation of araC gene causes the promoters (pBAD) and pC to become inactive, permanently repressed.

11. Similarities between lactose- and arabinose-operon : • Both of them contains three structural genes. • Inducible • When glucose is present at high levels it prevents both the synthesis of arabinose and lactose. • The function of both the operons are parallel. they undergo positive and negative regulation. • The products formed in both the operons are similar, positive regulation : sugars are formed negative regulation : no sugar

12. Thank you…..