Bookshelf

(A) A typical diauxic growth curve, as seen when Escherichia coli is grown in a medium containing a mixture of glucose and lactose. During the first few hours the bacteria divide exponentially, using the glucose as the carbon and energy source. When the glucose is used up there is a brief lag period while the lac genes are switched on before the bacteria return to exponential growth, now using up the lactose. (B) Glucose overrides the lactose repressor. If lactose is present then the repressor detaches from the operator and the lactose operon should be transcribed, but it remains silent if glucose is also present. Refer to Figure 9.24B for details of how the lactose repressor controls expression of the lactose operon. (C) Glucose exerts its effect on the lactose operon and other target genes by controlling the activity of adenylate cyclase and hence regulating the amount of cAMP in the cell. The catabolite activator protein (CAP) can attach to its DNA-binding site only in the presence of cAMP. If glucose is present, the cAMP level is low, so CAP does not bind to the DNA and does not activate the RNA polymerase. Once the glucose has been used up, the cAMP level rises, allowing CAP to bind to the DNA and activate transcription of the lactose operon and its other target genes.