Microbial Genetics Regulation Of Gene Expression

There might be millions of genes available in a bacterial cell. But all of them do not code at same time. Some wait for some special conditions to express themselves. Here question arise how it happen. i.e. right gene expression occur at right time.

When we get in deeper, we surprised that there is a foolproof mechanism available to control gene expression. And by which right gene expression tiered at the right time.

For example, the gram-positive soil bacterium Bacillus subtilis senses that the nutrient levels in its environment are decreasing, and it must determine if it should initiate sporulation. Escherichia coli cell is in an environment rich in carbon and energy sources, and it must determine which to use and when to use them. A pathogen is transmitted from a stream to the intestinal tract of its animal host, and it must adjust to the warmer temperature, increased nutrient supply, and defenses of the host.

These are some of the examples in which microbes must adjust themselves to survive, and this all happen ultimately by gene expression.

Regulation Of Transcription Initiation

Induction and repression are historically important, as they were the first regulatory processes to be understood in any detail. To understand regulation of transcription initiation, we must go through the induction and repression of enzyme synthesis. Because, finally it is enzyme, which express any gene. If enzyme is removed from the cell, there might be something which inhibit the expression of its gene.

Induction And Repression Of Enzyme Synthesis

Almost all the time many enzymes are produced with in the cell, because there might be several metabolic pathways are on to production energy and to perform some other tasks. Function of such enzyme may be termed as “housekeeping functions” and the genes that encode them are often referred to as housekeeping genes. Those housekeeping genes those are expressed continuously are said to be constitutive genes. Many genes, however, are expressed only when needed. The beta-galactosidase enzyme is an example of a regulated gene.

Beta-galactosidase is an inducible enzyme, means it is synthesized when required.  Beta-galactosidase is an enzyme required in catabolic pathway and many catabolic enzyme are inducible enzymes.

The genes for enzymes involved in the biosynthesis of amino acids and other substances, on the other hand, are often called repressible enzymes.  For instance, an amino acid present in the surroundings may inhibit the formation of enzymes responsible for its biosynthesis. The microorganisms does not need the biosynthetic enzymes for a particular substance if it is already available.

Inducible enzymes, in contrast, are required only when their substrate is available; they are missing in the absence of the inducer.

Variation in enzyme levels could be due to changes in the rates of enzyme degradation, most enzymes are relatively stable in growing bacteria. Induction and repression result principally from changes in the rate of transcription. 

When E. coli  growing in the absence of lactose, it lacks mRNA molecules coding for the synthesis of beta-galactosidase. In the presence of lactose, however, each cell has 35 to 50 beta-galactosidase mRNA molecules. The synthesis of mRNA is dramatically influenced by the presence of lactose.

Control Of Transcription Initiation By Regulatory Proteins

There are some regulatory proteins found in the cell. Function of it is induction and repression of gene. Regulatory proteins can exert either negative or positive control. Negative transcriptional control occurs when the protein inhibits initiation of transcription. Regulatory proteins that act in this fashion are called repress or proteins. positive transcriptional control occurs when the protein promotes transcription initiation. These proteins are called activator proteins.Repressor and activator proteins usually act by binding DNA at specific sites. 

Reference: Prescott Microbiology