Question

he lac operon in E. coli regulates the co-expression of three genes needed for the degradation of lactose (but absence of glucose) in the growth medium. These genes are not expressed when the bacteria has no lactose to feed on. How does the cell prevent expressing the lac operon in the absence of lactose? How does the presence of lactose activate the transcription of these three genes? (You do not need to explain the glucose level regulation.)

Answer 1

Lactose is a disaccharide made up of D-galactose and D-glucose. The enzyme beta - galactosidase breaks down lactose into galactose and glucose and also catalyzing the isomerization of lactose to allolactose, a compound important for regulating expression of the lac operon. Because allolactose, not lactose, is the inducer molecule, responsible for the increased production of the three enzymes of lac operon. In absence of lactose, allolactose will not be produced and the lac repressor bind to the operator and prevent transcription of mRNA (structural genes). RNA polymerase cannot bind to the promoter when repressor is bound to the operator. The mRNAs for the enzyme have a short half life, so the transcripts must be made continuously in order for the enzymes to be produced.

When lactose is present as the sole carbon source, some lactose is converted into allolactose by the enzyme beta galactosidase. Allolactose binds to the lac repressor and changes its shape (this is called allosteric shift). As a result, the repressor loses its affinity for the operator and dissociate from the site and the RNA polymerase bind to the promoter and induces the production of the lac operon - encoded enzymes.