Question

A bacterial strain expresses a mutant lac repressor protein that retains its ability to bind to the operator but cannot bind lactose. What is the effect on gene expression in these mutants? What happens when lactose is added to the growth medium?

Answer 1

A repressor is a DNA- or RNA binding protein that inhibits the expression of one or more genes by binding to the operator.The lac repressor binds tightly to the operator,which prevent transcription of the genes into messenger RNA - A DNA-binding repressor blocks the attachment of RNA polymerase which in turn prevents transcription of genes into mRNA.

And this repressor partially overlaps with the promoter when bond repressor get in RNA polymerase way and keeps it from transcribing the operon.

when lactose is added to the growth medium the repressor looses its ability to bind DNA.

it floats off the operator ,clearing the way for RNA polymerase to transcribe the operon this is due to the small molecule allolactose and the isomers of lactose. allolactose binds to the lac repressor and makes it change shape so it can nolonger bind to DNA.