The Regulator Gene

The regular gene directs the synthesis of a protein, which may be an active repressor or an inactive repressor (aporepressor). In the inducible system the active repressor formed has an affinity for the operator gene. In the absence of inducer the lac repressor protein binds to the operator gene and blocks the path of RNA polymerase. Thus the structural genes are unable to transcribe mRNA, and proteins synthesis does not take place. In the presence of an inducer, the repressor protein binds to the inducer to form an inducer-repressor-complex. Each repressor subunit has one binding site for the inducer. The repressor undergoes a conformational change which makes it inactive. It cannot now bind to the operator gene, and hence the structural genes can synthesize proteins.

In the repressible system the repressor protein formed by the regulator gene is inactive and does not block the operator. The structural genes, therefore synthesis proteins. The repressor-corepressor complex blocks the operator gene. The structural genes, therefore, cannot synthesize proteins. The repressor is activated in the presence of a corepressor, and the repressor-corepressor complex blocks the operator gene. The structural genes, therefore, cannot synthesize proteins.

In a lac operon of E.coli the repressor protein produced by the regulator gene consists of four subunits, each of 40,000 dlatons, and has 347amino acid residues. The active repressor protein has a strong affinity for a segment of the operator gene composed of 12-15 nucleotides.