<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> <P> The lac gene and its derivatives are amenable to use as a reporter gene in a number of bacterial - based selection techniques such as two hybrid analysis, in which the successful binding of a transcriptional activator to a specific promoter sequence must be determined . In LB plates containing X-gal, the colour change from white colonies to a shade of blue corresponds to about 20--100 β - galactosidase units, while tetrazolium lactose and MacConkey lactose media have a range of 100--1000 units, being most sensitive in the high and low parts of this range respectively . Since MacConkey lactose and tetrazolium lactose media both rely on the products of lactose breakdown, they require the presence of both lacZ and lacY genes . The many lac fusion techniques which include only the lacZ gene are thus suited to X-gal plates or ONPG liquid broths . </P>

The order of the structural genes in the lac operon is