Dna

Active repressor

Lactose present

|lj Lactose induces transcription by binding to the repressor, which cannot then bind to the operator. RNA polymerase binds to the promoter.

The repressor protein encoded by gene i prevents transcription by binding to the operator.

Active repressor

RNA polymerase cannot bind to the promoter; transcription is blocked.

No mRNA is produced, so no enzyme is produced.

RNA polymerase cannot bind to the promoter; transcription is blocked.

No mRNA is produced, so no enzyme is produced.

Lactose present

|lj Lactose induces transcription by binding to the repressor, which cannot then bind to the operator. RNA polymerase binds to the promoter.

Operator Rna

Enzymes of the lac operon pathway

ß-galactosidase Transacetylase

13.17 The lac Operon: An Inducible System Lactose (the inducer) leads to enzyme synthesis by preventing the repressor protein (which would have stopped transcription) from binding to the operator.

Enzymes of the lac operon pathway ft

ß-galactosidase Transacetylase

13.17 The lac Operon: An Inducible System Lactose (the inducer) leads to enzyme synthesis by preventing the repressor protein (which would have stopped transcription) from binding to the operator.

a lie close to the operon that it regulates, but some other regulatory genes are distant from their operons. Like all other genes, the i gene itself has a promoter, which can be designated pi. Because this promoter does not bind RNA poly-merase very effectively, only enough mRNA to synthesize about ten molecules of repressor protein per cell per generation is produced. This quantity of the repressor is enough to regulate the operon effectively—to produce more would be a waste of energy. There is no operator between p( and the i gene. Therefore, the repressor of the lac operon is a constitutive protein; that is, it is made at a constant rate that is not subject to environmental control.

Let's review the important features of inducible systems such as the lac operon:

► In the absence of inducer, the operon is turned off.

► Control is exerted by a regulatory protein—the repressor—that turns the operon off.

► Regulatory genes produce proteins whose sole function is to regulate the expression of other genes.

► Certain other DNA sequences (operators and promoters) do not code for proteins, but are binding sites for regulatory or other proteins.

► Adding inducer turns the operon on.

Operator-repressor control that represses transcription: The trp operon

We have seen that E. coli benefits from having an inducible system for lactose metabolism. Only when lactose is present does the system switch on. Equally valuable to a bacterium is the ability to switch off the synthesis of certain enzymes in response to the excessive accumulation of their end products. For example, if the amino acid tryptophan, an essential constituent of proteins, is present in ample concentration, it is advantageous to stop making the enzymes for tryptophan synthesis. When the synthesis of an enzyme can be turned off in response to such a biochemical cue, the enzyme is said to be repressible.

In repressible systems, the repressor protein cannot shut off its operon unless it first binds to a corepressor, which may be either the metabolic end product itself (tryptophan in this case) or an analog of it (Figure 13.18). If the end product is absent, the repressor protein cannot bind to the operator, and the operon is transcribed at a maximum rate. If the end product is present, the repressor binds to the operator, and the operon is turned off.

The difference between inducible and repressible systems is small, but significant:

► In inducible systems, the substrate of a metabolic pathway (the inducer) interacts with a regulatory protein (the

Tryptophan absent

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