What is Prokaryotic Gene Expression ?

Control of prokaryotic gene expression occurs primarily at the level of transcription. Prokaryotic genes are arranged in units consisting of a regulator gene and associated units called operons. An operon consists of a promotor site, an operator site, and structural genes. The regulator gene, adjacent to the promotor and operator sites, produces repressors, protein molecules that bind to the operator site, preventing the attachment of RNA polymerase and the start of transcription of structural genes.

The concentration of certain enzymes, called inducible enzymes, is very much dependent upon the concentration of substrate in the cell. For example, in E. coli, the concentration of beta-galactosidase enzyme is very low in a lactose-free environment, but a large increase of the enzyme proportional to the lactose added to the growth medium is produced by a gene segment called the lac operon.

In high concentrations of lactose, inducer sugar molecules bind to the lac repressors and detach them from the operator, allowing RNA polymerase to bind and protein synthesis to occur. Two other proteins are produced along with beta-galactosidase - a permease and a transacetylase. The beta-galactosidase cleaves lactose into glucose and galactose; the permease facilitates transport of lactose across the bacterial cell membrane. The function of the transacetylase is not clear. When the lactose level decreases, repressors bind to the operator site and the operon becomes inactive.