The role of a metabolite that controls a repressible operon is toa) bind to the promoter region and decrease the affinity of rna polymerase for thepromoter. b) bind to the repressor protein and inactivate it. c) bind to the operator region and block the attachment of rna polymerase to the promoter. d) increase the production of inactive repressor proteins. e) bind to the repressor protein and activate it.

Ans. (E) bind to the repressor protein and activate it.

Repressible operon is defined as operon that normally remains active, but can be inhibit by a repressor molecule (a protein). When transcription of genes of a particular repressible operon is not required, a co-repressor( metabolite) molecule binds and activates the repressor molecule. The repressor protein, then binds to the operator and inhibits gene expression.

For example, tryptophan (tryp) operon is a repressible operon, responsible for the gene expression of enzymes for tryptophan synthesis. When tryptophan is present in sufficient amount, expression of tryp operon is not required.

The tryptophan (metabolite) itself, acts as co-repressor and activates repressor protein for tryp operon and inhibits expression of genes.

Thus, the correct answer is option (E).

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