The induction of gene expression, such as we have just described for the lac operon, generally relates to catabolic (breakdown) reactions. Anabolic (synthetic) reactions, such as those leading to the production of specific amino acids, by contrast, are often controlled by the repression of key genes.
Enzyme repression mechanisms operate along similar lines to induction mechanisms, but the determining factor here is not the substrate of the enzymes in question (lactose in our example), but the end-product of their action. The trp operon contains a cluster of genes encoding five enzymes involved in the synthesis of the amino acid tryptophan. (Figure 11.16) In the presence of tryptophan, the cell has no need to synthesise its
Glucose is central to the reactions of glycolysis (Chapter 6), and is utilised by E. coli with high efficiency, because the enzymes involved are permanently switched on or constitutive. The j-galactosidase required for lactose breakdown, however, must be induced. What happens then, when E. coli is presented with a mixture of both glucose and lactose? It would be more efficient to metabolise the glucose, with the ready-to-use enzymes, but from what you have learnt elsewhere in this section (see Figure 11.15b), the presence of lactose would induce formation of j-galactosidase and subsequent lactose breakdown, a less energy-efficient way of going about things. In fact, E. coli has a way of making sure that while the readily utilised glucose is present, it takes precedence. It does this by repressing the formation of j-galactosidase, a phenomenon known as catabolite repression. Thus, the presence of a 'preferred' nutrient prevents the synthesis of enzymes needed to metabolise a less favoured one.
This is because glucose inhibits the formation of cAMP, which is required for the binding of the CAP to its site on the lac promoter. When glucose levels drop, more cAMP forms and causes CAP to bind to the CAP binding site. Thus, after a delay, the enzymes needed for lactose catabolism are synthesised, and the lactose is utilised, leading to a diauxic growth curve (see Chapter 5).
Figure 11.16 The trp operon. Five structural genes A-E encode enzymes necessary for the synthesis of tryptophan. (a) In the absence of tryptophan, transcription of the operon proceeds unhindered. Although a repressor protein is produced, it is inactive and unable to bind to the operator sequence. (b) Tryptophan activates the repressor by binding to it. This prevents RNA polymerase from binding to the promoter, and transcription is blocked
Figure 11.16 The trp operon. Five structural genes A-E encode enzymes necessary for the synthesis of tryptophan. (a) In the absence of tryptophan, transcription of the operon proceeds unhindered. Although a repressor protein is produced, it is inactive and unable to bind to the operator sequence. (b) Tryptophan activates the repressor by binding to it. This prevents RNA polymerase from binding to the promoter, and transcription is blocked own, so the operon is switched off. This is achieved by tryptophan binding to and activating a repressor protein, which in turn binds to the operator of the trp operon and prevents transcription of the synthetic enzymes. The tryptophan here is said to act as a corepressor. As tryptophan is used up and its level in the cell falls, the repressor reverts to its inactive form, allowing transcription of the tryptophan-synthesising enzymes to go ahead unhindered.
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