Constitutive, Inducible, and Repressible Gene Expression
Types of genes depending on how they are regulated:
Constitutive gene: gene that is transcribed continually
Facultative gene: gene only transcribed when needed
- Housekeeping gene is a typically constitutive gene that is transcribed relatively at a constant level.
- Housekeeping genes are required for the maintenance of basic cellular function, and are expressed in all cells of an organism under normal and patho-physiological conditions.
- These genes specify cellular components that perform housekeeping functions.
- Examples of housekeeping genes products include tRNA molecules, rRNA molecules, ribosomal proteins, actin, ubiquitin, RNA polymerase subunits and enzymes catalyzing metabolic processes.
(If the gene product is an enzyme that is produced continuously, regardless of the chemical makeup of the environment, they were called constitutive enzymes)
- A gene transcribed only when their products are required for growth or when a cell receives a signal from its surroundings.
- These gene products are required only under certain environmental conditions. Constitutive synthesis of such gene products would be wasteful. This energy can be utilized for more rapid growth.
- Facultative genes can be inducible or repressible in responsive to environmental change or dependent on the position in the cell cycle.
These genes can be repressed – turned off
normally they are expressed but may be turned off if not needed
Genes that encode enzymes involved in anabolic pathways (Biosynthesis of amino acids, purines, pyrimidines, and vitamins) usually are turned off in the presence of the end product of the pathway.
Example: structural genes of Trp operon
The E. coli genome contains five genes encoding enzymes that catalyze steps in the biosynthesis of tryptophan. These five genes must be expressed in E. coli cells growing in an environment devoid of tryptophan in order to provide adequate amounts of this amino acid for ongoing protein synthesis.
When E. coli cells are present in an environment containing enough tryptophan to support optimal growth, the continued synthesis of the tryptophan biosynthetic enzymes would be a waste of energy. Thus, a regulatory mechanism has evolved in E. coli that turns off the synthesis of the tryptophan biosynthetic enzymes when external tryptophan is available.
Repressible genes are transcriptionally regulated either by a co repressor or an inhibitor
Co repressor: A small effector molecule which binds to a repressor protein that allows the binding of the repressor protein to DNA (Effector molecules involved in repression of gene expression)
Inhibitor: A small effector molecule which binds to an activator protein that restricts the binding of the activator protein to the DNA
The process by which a gene’s expression is turned off is called repression.
When the expression of this repressed gene is turned on, it is said to be “derepressed” and such a response is called derepression.
- The process of turning on the expression of genes in response to a substance in the environment is called induction.
- These genes can be induced – turned on
normally not expressed but may be turned on if conditions warrant
- If the product of the inducible gene is an enzyme a, they were called inducible enzymes
Example: Structural genes of Lac operon
In natural environments (intestinal tracts and sewers), E. coli cells probably encounter an absence of glucose and the presence of lactose relatively infrequently. Therefore, the E. coli genes encoding the enzymes involved in lactose utilization are probably turned off most of the time.
If cells growing on a carbohydrate other than lactose are transferred to medium containing lactose as the only carbon source, they quickly begin to synthesize the enzymes required for lactose utilization
(Genes that encode enzymes involved in catabolic pathways often are expressed only in the presence of the substrates of the enzymes)
Inducible genes are transcriptionally regulated by an inducer. The effector molecules involved in induction of gene expression are called inducers. The effectors are usually small molecules such as amino acids, sugars, and similar metabolites.
Inducer is a small effector molecule that binds to:
1) An activator protein (allows the binding of activator to DNA)
2) A repressor protein (hinders the binding of repressor to DNA)
- Induction occurs at the level of transcription. Induction alters the rate of enzyme synthesis, not the activity of existing enzyme molecules. Induction should not be confused with enzyme activation, which occurs when the binding of a small molecule to an enzyme increases the activity of the enzyme, but does not affect its rate of synthesis.
- Repression occurs at the level of transcription. Repression should not be confused with feedback inhibition, which occurs when the product of a biosynthetic pathway binds to and inhibits the activity of the first enzyme in the pathway, but does not affect the synthesis of the enzyme.