GENETICS bacterial gene regulation The lac operon One of t

GENETICS - bacterial gene regulation - The lac operon

One of the original mutations isolated by Jacob and Monod was O⁰ (also called O^-), often caused by single base pair changes. The O⁰ mutant fails to express B-gal and permease under any condition. Although we now connect the symbol O, operator designation to the binding site of the lac repressor, originally the O locus encompassed the whole lac promoter region. It was shown later that the O⁰ mutant can bind both CAP and the lacI product. What positive regulator was affected by the O⁰ mutation and what could a single bp change affect in this mutant?

A) Binding by RNA polymerase to the lac promoter region. The base pair could disrupt the

-35 or -10 consensus regions.

B) The lacI coding sequence. The allolactose binding site.

C) The rho terminator. The termination site for the operon.

D) The lacI gene promoter. The Shine-Delgarno sequence

E) The ribosome. The Shine Delgarno site.

Please explain!

Solution

A and E are the two positive regulators that might be affected by the O0 mutation with a single base pair change.

As the O0 mutant is found to be able to bind to CAP and LacI, base pair change in repressor, terminator, LacI promoter should be able to express the genes. So, the other options cannot be the reasons for the current situation.

A)

If the binding of RNA polymerase to the lac promoter region is disrupted by change in a single base pair at the -35 or -10 regions, the transcription of the permease and B-gal is stopped. So, A is correct.

E)

Another reason for the genes to not get expressed can be a mutation in the ribosome or the shine Delgarno sequence which can interrupt in the translation process of these enzymes. So, E is correct.