Info

^ntc lac repressor lacZ

No mRNA transcription

+ lactose + glucose (low cAMP)

+ lactose

- glucose (high cAMP)

+ lactose + glucose (low cAMP)

+ lactose

- glucose (high cAMP)

▲ FIGURE 4-16 Regulation of transcription from the lac operon of E. coli. (Top) The transcription-control region, composed of =100 base pairs, includes three protein-binding regions: the CAP site, which binds catabolite activator protein; the lac promoter, which binds the RNA polymerase-a70 complex; and the lac operator, which binds lac repressor. The lacZ gene, the first of three genes in the operon, is shown to the right. (a) In the absence of lactose, very little lac mRNA is produced because the lac repressor binds to the operator, inhibiting transcription initiation by RNA polymerase-a70 (b) In the presence of glucose and lactose, lac repressor binds lactose and dissociates from the operator, allowing RNA polymerase-a70 to initiate transcription at a low rate. (c) Maximal transcription of the lac operon occurs in the presence of lactose and absence of glucose. In this situation, cAMP increases in response to the low glucose concentration and forms the CAP-cAMP complex, which binds to the CAP site, where it interacts with RNA polymerase to stimulate the rate of transcription initiation.

mRNA and subsequently of the enzymes encoded by the lac operon (Figure 4-16c).

Although the promoters for different E. coli genes exhibit considerable homology, their exact sequences differ. The promoter sequence determines the intrinsic rate at which an RNA polymerase-a complex initiates transcription of a gene in the absence of a repressor or activator protein. Promoters that support a high rate of transcription initiation are called strong promoters. Those that support a low rate of transcription initiation are called weak promoters. The lac operon, for instance, has a weak promoter; its low intrinsic rate of initiation is further reduced by the lac repressor and substantially increased by the cAMP-CAP activator.

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