<Li> When lactose is present but a preferred carbon source (like glucose) is also present then a small amount of enzyme is produced (Lac repressor is not bound to the operator). </Li> <Li> When glucose is absent, CAP - cAMP binds to a specific DNA site upstream of the promoter and makes a direct protein - protein interaction with RNAP that facilitates the binding of RNAP to the promoter . </Li> <P> The delay between growth phases reflects the time needed to produce sufficient quantities of lactose - metabolizing enzymes . First, the CAP regulatory protein has to assemble on the lac promoter, resulting in an increase in the production of lac mRNA . More available copies of the lac mRNA results in the production (see translation) of significantly more copies of LacZ (β - galactosidase, for lactose metabolism) and LacY (lactose permease to transport lactose into the cell). After a delay needed to increase the level of the lactose metabolizing enzymes, the bacteria enter into a new rapid phase of cell growth . </P> <P> Two puzzles of catabolite repression relate to how cAMP levels are coupled to the presence of glucose, and secondly, why the cells should even bother . After lactose is cleaved it actually forms glucose and galactose (easily converted to glucose). In metabolic terms, lactose is just as good a carbon and energy source as glucose . The cAMP level is related not to intracellular glucose concentration but to the rate of glucose transport, which influences the activity of adenylate cyclase . (In addition, glucose transport also leads to direct inhibition of the lactose permease .) As to why E. coli works this way, one can only speculate . All enteric bacteria ferment glucose, which suggests they encounter it frequently . It is possible that a small difference in efficiency of transport or metabolism of glucose v. lactose makes it advantageous for cells to regulate the lac operon in this way . </P>

The e coli lac operon has how many operator sequences