Question

A newly identified bacterial strain needs nedine as a growth factor. To synthesize nedine these bacteria use three enzymes that are encoded by the genes nedA, nedB and nedC in the ned operon. The expression of these three genes is controlled by a regulatory protein NedR. Nedine binds to NedR, and it changes the ability of NedlR to bind to the regulatory region of the ned operon. The ned genes are expressed only in the absence of nedine. The regulator NedR, only when bound to nedine, can bind at the regulatory region of the ned operon.

Is the NedR a positive or negative regulator? (One answer has mentioned positive whilst another mentions negative??)

Answer 1

Ans

NedR is a negative regulator. The operon is negatively controlled because the binding of the regulatory protein NedR will inhibit the expression of genes of operon. This can be confirmed from the fact that NedR will bind to regulatory region of operon only when it is bound to nedine. And presence of nedine will inhibit gene expression. This indicates that NedR is a repressor and is inactive by itself (aporepressor). It will not bind to regulatory region. It becomes active by combining with nedine. In the absence of nedine, it has no effect on operon. Thus the genes will express and produce Enzymes to synthesise nedine. Nedine, here acts as an effector to convert the aporepressor NedR into active repressor and hence nedine is a corepressor. Activation of repressor will lead to shutting off the structural genes by binding to the regulatory site.