Gene Regulation in Prokaryotes

Bacteria cells can regulate the synthesis of enzymes in such a way that the enzymes are produced only when the substrates which these enzyme utilise are present in the cell.

Lactose Operon

In 1961, two French microbiologists, Francois Jacob and Jacques Monod proposed a mechanism, called Operon Hypothesis, for the regulation of gene action in E. coli. It got them the Nobel Prize in 1965.

fig. 23.1 - Francois Jacob and Jacques Monod with their Colleague

They found that the addition of lactose to the culture induced the formation of three enzymes: -galactosidase, lactose permease and transacetylase, which break down lactose to galactose and glucose so that lactose may be utilized by the bacterium. The genes which code for these enzymes lie adjacent to each other in the chromosome and are called structural genes, or cistrons. They are transcribed simultaneously by RNA polymerase into a single long mRNA chain which has codons for all the three enzymes. Such an mRNA transcribed from many genes is described as Polycistronic. The functioning of these cistrons is controlled by a stretch of DNA called operator, located next to the cistron Z. The cistrons synthesize mRNA when the operator switch is turned on and stop the synthesis when the operator switch is turned off. When bound by a repressor, operator is turned off, and when repressor is not bound, it is turned on. The repressor is a protein, which is produced by a regulator gene (RG). A part of DNA that comprises operator and structural genes and functions as a regulating unit in transcription is called operon. The operon works in cooperation with a promotor gene, a regulator gene, a repressor protein and an inducer or corepressor substances. There are two types of operons: inducible and repressible.