Genetic Regulation of Prokaryotic Cells (World of Microbiology and Immunology)
The ability of a bacterium to regulate the expression of the myriad of genes contained in the chromosome and plasmids is essential to the growth and survival of the microorganism. While a bacterium has many genes that code for a variety of proteins, these genes are not all expressed at the same time. Some genes are active all the time, while others are active only at specific times in the growth cycle of the bacterium or in response to a certain environmental condition. The amounts of proteins that are produced are not all the same. Moreover, the triggers that stimulate the expression of one gene can be quite different from the triggers for another gene. The ability of a prokaryotic cells (bacterial are the prototypical system) to orchestrate the expression of the repertoire of genes constitutes genetic regulation.
The activity of genes in the manufacture of compounds by the bacterium, such as in the biosynthetic pathways of the microbe, is often under a type of control known as feedback inhibition. In this type of genetic regulation, the object of the regulation is the first enzyme that is unique to the pathway (not to the gene that coeds for the enzyme). In biosynthetic pathways, there are typically a number of compounds that can be formed in the various enzymatic reactions within the pathway. Feedback inhibition occurs when the final product inhibits the first biosynthetic enzyme. Blocking the first enzymatic step prevent the remainder of the enzymes from having any material on which to act.
Feedback inhibition is possible because the biosynthetic enzymes have two binding regions. If both sites are occupied by the end product, the three-dimensional structure of the enzyme is changed such that it cannot bind any more of the protein it is supposed to enzymatically alter. But, when the amount of the end product decreases, one of the enzyme's binding sites is no longer occupied, and the enzyme can resume its function. The function of the enzyme can also be affected by modifying the structure of side groups that protrude from the enzyme molecule. This alteration is also reversible, when the concentration of the blocking end product is lowered.
Feedback inhibition is a genetic regulatory mechanism that allows a bacterium to rapidly respond to changes in concentration of a particular compound. The bacterium does not have to manufacture protein, as the molecules already exist and are primed to resume activity once conditions are favorable.
Regulation also operates directly at the level of the genes. This type of genetic regulation is called induction (when the gene is stimulated into action) or repression (when the gene's activity is reversible silenced). Regulating the activity of genes, rather than the activity of the proteins made by the genes can save a bacterium the energy of manufacturing the protein.
Induction and repression depend on the binding of a molecule known as RNA polymerase to regions that signal the beginning of a stretch of DNA that code for proteins. The three-dimensional shape of the polymerase-binding region influences the binding of the RNA polymerase. The binding of molecules called effectors can in turn influence the shape of this region. If an effector alters the shape of the polymerase-binding region so that the polymerase is able to bind, the effect is called induction. If the effector binding prevents the polymerase from binding, then the effect is known as repression.
Induction and repression tend to cycle back and forth, in response to the level of effector, and so in response to whatever environmental or other condition the particular effector is sensitive to. A visual analogy would the turning on and off of room light under the control of a very sensitive light meter, as clouds obscured the sunlight from one moment to the next.
Another genetic regulatory mechanism that operates only in procaryotes is termed attenuation. Attenuation requires a close coupling between the synthesis of ribonucleic acid from a DNA template (transcription) and the use of the RNA as another template to manufacture protein (translation). These processes are very closely coupled in procaryotes, particularly in the activity of enzymes that participate in the making of amino acids.
In the process of attenuation, a gene that codes for an enzyme required for the synthesis of an amino acid is not active until the level of that amino acid lowers to some threshold level. At this level, the molecules called ribosomes physically stall as they move down the beginning of the RNA template that encodes protein. The stalling prevents the formation of a signal that otherwise stops the onward movement of the ribosomes. After the pause, because the stop signal has not formed, the ribosomes resume their movement and the protein is produced. When the level of the critical amino acid is higher, the ribosomes do not stall, encounter a stop signal, and the synthesis of the protein does not occur.
These processes operate simultaneously for many genes in a bacterium. For some of these genes, the controlling factors are independent of one another. But for other proteins, a common factor, such as a sensory protein that can sense changes in the environment and provide a signal to the various regulatory processes, also operates. This genetic regulation pattern is referred to as global regulation. An example of global regulation is a phenomenon called diauxic growth, which is exemplified by the lactose operon (also called the lac operon). Diauxic growth allows a bacterium to preferentially utilize one nutrient (such as glucose) when two nutrients (such as glucose and lactose) are present. When the preferred source is exhausted, metabolism can switch so as to utilize the second source (lactose). This nutrient preference involves genetic regulation of protein production.
Other genetic regulatory mechanisms operate in response to fluctuations in temperature, pH, oxygen level, the attraction or repulsion of a bacterium from a compound (chemotaxis), and the production of a spore.
See also Cell cycle (prokaryotic), genetic regulation of; Microbial genetics