Question

The lac operon is just one specific example of an operon that is regulated both negatively, by binding of a repressor, and positively by catabolite repression.  Describe another example of this type, providing details of the ligands that positively and negatively regulate transcription of the operon, and briefly describe the genes in the operon and the molecular logic of the specific operon you describe.

Answer 1

Solution:-

Arabinose operon

• E. coli bacteria need arabinose as an energy and carbon source
• Arabinose is a five carbon sugar and needs to convert into Ribulose-5-phosphate before it can be metabolized.
• Arabinose operon has three structural genes araB, araA and araD that encodes enzymes, which carry out this conversion of arabinose to ribulose-5-phosphate.
• The operon also has araC gene which code for C protein (regulatory protein)

Arabinose operon structure

araC - codes for C protein

PC - promoter for araC

O2- operator for araC

O1- operator for araB, araA and araD

I1- initiator 1 for araB,araA and araD

I2-  initiator 1 for araB,araA and araD

CAP- CAP-cAMP binding site

araB - codes for ribulokinase

araA- codes for L-arabinose isomerase

araD- codes for L-ribulose-5-phosphate 4-epimerase

Negative control of arabinose operon

• in the absence of arabinose the dimers of C protein binds to O2, O1 and I1.
• This  forms a DNA loop which effectively block the transcription of the genes araB, araA and araD.

Positive control of arabinose operon

• In the presence of arabinose, araC encodes C protein, binds with arabinose and undergo a conformational change that enables C protein to binds with O2 and I2 also.
• This result in the formation of a different loop and results in the transcription of araC gene and hence more C proteins (autoregulation of C protein).
• In the absence of glucose cAMP-CAP is formed which binds to CAP site.
• C protein binds with I1 and I2 sites interact with bound CAP enables the RNA polymerase to initiate transcription of the arabinose operon promoter (O1).