Regulation of Gene Expression

Genes of a cell are expressed to perform different functions. For example E. coli synthesises the enzyme beta-galactosidase, which hydrolyses lactose into galactose and glucose:

Lactose + H2O → (beta-galactosidase) → Galactose + Glucose

If E. coli has no lactose in its surrounding medium as a source of energy, beta-galactosidase is not synthesised: it is the metabolic, physiological or environmental conditions that regulate the expression of genes. The development and differentiation of an embryo into an adult organism is likewise the result of the coordinated regulation of several sets of genes. One has to understand the mechanism by which organisms regulate gene expression in response to changes in the environment, and whether one mechanism serves all genes or different genes are regulated differently.

Certain bacteria like E. coli adapt to their chemical environment by synthesising particular enzymes depending on the substrate present; such adaptive enzymes are inducible enzymes. A set of genes is switched on when a new substrate has to be metabolised - this phenomenon is induction, the small molecule responsible is the inducer, and this is positive control.

Gene expression is the multistep process by which a gene is regulated and its product synthesised, ending in the formation of a polypeptide. It is regulated at different levels. In eukaryotes the regulation can be at:

1. the transcriptional level (formation of the primary transcript);
2. the processing level (regulation of splicing);
3. the transport of mRNA from the nucleus to the cytoplasm;
4. the translational level.