When glucose levels decline in the cell, accumulating cAMP binds to the positive regulator catabolite activator protein CAPa protein that binds to the promoters of operons that control the processing of alternative sugars.
The discovery of a new class of genetic elements, regulatory genes, the demonstration that the expression of both lacZ and lacY genes is negatively regulated by a repressor, and, finally, the hypothesis of an obligatory intermediate, the messenger ribonucleic acid mRNAin the transfer of structural information from gene to protein, culminated in the concept of the operon.
This leads to the mRNA pairing that prevents transcription and translation of the rest of the operon. Copyright IOS Press and the authors.
Furthermore, the development of galactoside analogues, mainly thiogalactosides, which served to dissociate enzyme activity from enzyme induction gratuitous inducers permitted a straightforward analysis of the system. For this work, they won the Nobel Prize in Physiology or Medicine in The secret to this response lies in a tiny protein, or peptide, encoded by the leader.
When tryptophan is plentiful, two tryptophan molecules bind the repressor protein at the operator sequence. Only when glucose is absent and lactose is present will the lac operon be transcribed.
This property of the Lac repressor has been used to screen complex peptide libraries for direct interaction with a given receptor. This could be one of the possible signal transduction mechanisms by which bacteria might modulate gene expression upon starvation stimuli. In contrast, there are other prokaryotic operons that are expressed only when needed and are regulated by repressors, activators, and inducers.