Regulation of Gene Expression in Prokaryotes

Regulation of Gene Expression in Prokaryotes - Bacteria such as E. coli are exposed to a wide variety of environmental conditions. Natural selection will preserve those organisms that have evolved ways of adapting to the wide range of environmental conditions encountered during their evolution.

The adaptability of bacteria and other prokaryotes depends on their ability to turn on and off the expression of specific sets of genes in response to the specific demands of the environment. The expression of a particular gene is turned on when the products of these genes are needed for growth in a given environment.

Their expression is turned off, when their products are no longer needed for growth in the existing environment. The synthesis of gene transcripts and translation products requires the expenditure of considerable energy.

By "turning off' the expression of genes when their products are not needed, an organism can avoid wasting energy and can utilize the conserved energy to synthesize products that maximize the growth rate.

For example, if a given metabolite is not present, enzymes for its breakdown are not necessary, and synthesizing these enzymes is wasteful If a cell produces enzymes for the degradation of a particular carbon source only when this carbon source is present in the environment, the enzyme system is known as an inducible system.

In contrast, the enzymes in many synthetic pathways are in low concentration or absent when an adequate quantity of the end product of the pathway is already available to the cell. That is, if the cell encounters an abundance of the amino acids, e.g. tryptophan in the environment or if it is over produced, the cell stops the manufacture of tryptophan until a need arises again.

A repressible system is a system of enzymes whose presence is repressed, stopping the production of the end product when it is no longer needed. Repressible systems are repressed by an excess of the end product of their synthetic pathway.Bacteria regulate or alter the gene expression either by using positive or negative regulation.

Positive and negative regulation look similar in most aspects, but the fundamental difference is what type of molecule, i.e., repressor alone, e.g. lac operon or repressor along with inducer/end product, e.g. trp operon, is binding to promoter, and also whether the molecule is increasing (inducer) or decreasing (repressor) the gene expression.