Types of Control Mechanisms

The control of protein synthesis by regulator proteins may be by induction or repression. There may also be negative or positive control.

Induction

A category or enzymes called inducible enzymes are normally absent from the cell or are present in very small quantities. The increase in quantity only in the presence of an inducer. Such a system is called an inducible system. A bacterial cultural of E.coli growing in a medium with glucose as the source of carbon produces only minute quantities of the enzyme β-galactosidase. Only one or two molecules of the enzyme are present. When lactose is added to the medium the production of β-galactosidase starts, and within two to three minutes about 3,000 molecules are synthesized by the Z gene. β-galactosidase hydrolyses lactose into the sugar galactose and glucose. Some galactose and glucose molecules are converted into allolactose, which is the inducer for β-galactosidase synthesis.

In the lac system allolacose is the actual inducer while lactose is the apparent inducer. An artificial inducer commonly used in experimental work is isopropylthiogalactoside (IPTG).

When the inducer is absent the active repressor protein produced by the regular gene associates with the operator gene and blocks it. No transcription of mRNA by the structural genes can therefore take place, and hence there is no enzyme synthesis. Absence of the inducer may be due fact that is not present in the growth medium or is not synthesized by the microorganism.

Repression

The synthesis of an enzyme may be inhibited by a product of metabolism. In E.coli trypotphan synthetase is inhibited by tryptophan. Thus tryptophan synthetase is formed only if the bacterium is grown in a tryptophan free medium. The amino acid histidine also acts as a repressor. This process is called enzyme repression. In the repressible system the protein produced by the regulator gene is an inactive repressor, called the aporepressor. The aporepressor on combining with a co repressor or effector molecule become activated. It undergoes a conformational change which enables which it to bind to the operator gene.

When the corepressor is absent the inactive aporepressor does not bind to the operator gene. Because the operator gene is not blocked, the structural genes transcribe mRNA, and protein synthesis takes place.

When the corepressor is present the aporepressor combines with it to form a complex, and is activated. The complex blocks the operator gene. The structural genes are therefore unable to synthesize mRNA, and consequently no protein synthesis takes place. The corepressor may be a product of one of the enzymes produced by the structural genes.