by Prof.Siddharth Sanghvi
Regulation of gene expression is a broad term that can occur at various levels within an organism. Considering that gene expression ultimately leads to the formation of a polypeptide (protein), its regulation can be exerted at several stages:
Genes are expressed to perform specific functions. For instance, E. coli synthesizes the enzyme beta-galactosidase to hydrolyze lactose into glucose and galactose, which are then used as energy sources. If lactose is not available, the bacteria do not need this enzyme. Thus, gene expression is largely regulated by metabolic, physiological, or environmental conditions. In prokaryotes, the primary control point for gene expression is the rate of transcriptional initiation.
In prokaryotes, the activity of RNA polymerase at a given promoter is regulated by interactions with accessory proteins (activators and repressors) that affect its ability to recognize start sites. The accessibility of promoter regions is often controlled by the interaction of proteins with sequences called operators. The operator region is typically adjacent to the promoter in most operons and binds a repressor protein.
Operon: A polycistronic structural gene (a gene that codes for multiple proteins) regulated by a common promoter and regulatory genes. This arrangement is very common in bacteria.
Examples of Operons: Lac operon, Trp operon, Ara operon, His operon, Val operon.
The elucidation of the Lac Operon by Francois Jacob and Jacque Monod was a landmark achievement, as they were the first to describe a transcriptionally regulated system.
The Lac Operon (where 'lac' refers to lactose) in E. coli consists of one regulatory gene and three structural genes:
These genes are responsible for the metabolism of lactose and are transcribed together as a single mRNA molecule:
In most operons, the genes present are needed together to function in the same or related metabolic pathway.
Lactose acts as the substrate for the enzyme beta-galactosidase, and it also functions as an inducer, regulating the switching on and off of the operon.
When lactose is present in the growth medium (and glucose is absent or low), it is transported into the bacterial cells via the action of permease. Inside the cell, a small amount of beta-galactosidase (which is always present, see next section) converts lactose into its isomer, allolactose.
Allolactose is the actual inducer that interacts with the repressor protein.
The regulation of the Lac Operon by the repressor protein is referred to as negative regulation.
A very low level of expression of the Lac Operon must always be present in the cell, even in the absence of lactose. This is crucial because:
In addition to negative regulation by the repressor, the Lac Operon is also subject to positive regulation, primarily influenced by the presence or absence of glucose.
The concentration of cAMP in the cell is inversely related to the concentration of glucose:
When glucose is scarce, cAMP levels rise. cAMP then binds to CAP, forming the cAMP-CAP complex.
This cAMP-CAP complex binds to a specific site near the promoter region of the Lac Operon. This binding significantly enhances the affinity of RNA polymerase for the promoter, leading to a much higher rate of transcription (activation) compared to when only negative regulation is relieved.
This positive control ensures that E. coli preferentially utilizes glucose when available, as glucose metabolism is more energy-efficient than lactose metabolism.
The expression of the Lac Operon is a finely tuned process that considers the availability of both glucose and lactose.
| Glucose Status | Lactose Status | Repressor Binding | CAP-cAMP Binding | Lac Operon Expression | Reason |
|---|---|---|---|---|---|
| Present (High) | Absent | Bound | No (low cAMP) | OFF | Repressor blocks transcription; no need for lactose enzymes. |
| Present (High) | Present | Not bound (inactivated by allolactose) | No (low cAMP) | Very Low (Basal) | Repressor is off, but CAP-cAMP complex doesn't form, so RNA polymerase affinity for promoter is low. Glucose is preferred. |
| Absent (Low) | Absent | Bound | Bound (high cAMP) | OFF | Repressor blocks transcription, even if CAP-cAMP is bound. No lactose to metabolize. |
| Absent (Low) | Present | Not bound (inactivated by allolactose) | Bound (high cAMP) | HIGH | Repressor is off, and CAP-cAMP enhances RNA polymerase activity. Lactose is the only energy source. |