bind to the operator when there is no more lactose to metabolize. Also as the availability of lactose gets low, the lactose molecules that were bound to repressor molecules can be degraded. This frees up repressor molecules. When the enzymes are no longer needed, transcription is shut down. Lactose alone cannot induce the lac operon. E. Coli prefers to utilize glucose rather than lactose because glucose is simpler and more abundant. The bacterium does not want to waste energy making lactose-metabolizing enzymes when there is plenty of glucose present. The lac operon should be induced only when there is lactose present and no glucose at all.The lac promoter by itself binds poorly to RNA polymerase. So even if the repressor is not bound to the operator, transcription does not readily occur. Therefore the presence of lactose alone does not induce transcription. When catabolite activator protein of CAP, an accessory protein binds to the promoter it stimulates the interaction of RNA polymerase and transcription. CAP binds to the promoter only when there is no glucose in the environment. When glucose levels are low, there is an increase in the level of cyclic AMP or camp, which is metabolic derivative of ATP. cAMP binds to CAP and then Cap binds to the lac promoter and transcription is stimulated. The lac operon will be transcribed only when lactose is present and there in no glucose.With a repressible operon we need a situation where the products of an operon are normally needed, and only under unusual circumstances would the operon need to be shut down. E coli normally has to make its own tryptophan. Tryptophan is rarely found in the environment. The biosyntheticf pathway for the production of tryptophan consists of five enzymatic steps. The coding regions for each other the enzymes in the pathway are located within the operon. The promoter of the trp operon is highly attracted to RNA polymerase. No additional protein is ...
