Question

In regulation of gene expression in prokaryotes, choose which among the following option is correct.
I) Lactose acts as the suppressor for gene expression.
II) Tryptophan acts as the inducer for gene expression.
III) Regulator gene is the one that produces the repressor molecule.

• A. Option I is correct.
• B. Option II is correct.
• C. Option III is correct.
• D. Option II and I are correct.
• E. Option II and III are correct.

Answer 1

The correct option is C Option III is correct.

The inducer is a molecule that binds to the repressor and causes conformational changes in repressor which in turn can no longer bind to the operator. In the presence of lactose, allolactose; an isomer of lactose, binds to the repressor and causes an allosteric shift. The altered repressor can not bind to the operator, allowing RNA polymerase to transcribe the lac genes. Thus lactose serves as inducer, not as suppressor; the statement I is wrong. Two tryptophan molecules bind to the trp repressor. This binding activates the inactive repressor to bind to the trp operator. Binding of the tryptophan-repressor complex at the operator prevents the RNA polymerase from binding the downstream genes and thus prevents their transcription. Here tryptophan serves as a corepressor and inhibits the expression of genes not by directly binding to DNA but instead indirectly regulates gene expression by binding to repressors. Statement II is wrong. Regulator gene regulates the expression of structural genes by its protein products that are mostly transcription factors. These transcription factors, also called as the repressor, bind to the operator region which is located just upstream the binding site of the RNA polymerase and hence prevent the binding and transcription. Statement III is correct.