In eukaryotes, gene expression occurs by following levels:
1. Transcriptional level (formation of primary transcript).
2. Processing level (splicing, capping etc.).
3. Transport of mRNA from nucleus to the cytoplasm.
4. Translational level (formation of a polypeptide).
The metabolic, physiological and environmental conditions regulate gene expression. E.g.
ú In E. coli, the beta-galactosidase enzyme hydrolyses lactose into galactose & glucose. In the absence of lactose, the synthesis of beta-galactosidase stops.
ú The development and differentiation of embryo into adult are a result of the expression of several set of genes.
If a substrate is added to growth medium of bacteria, a set of genes is switched on to metabolize it. It is called induction.
When a metabolite (product) is added, the genes to produce it are turned off. This is called repression.
§ “Each metabolic reaction is controlled by a set of genes”
§ All the genes regulating a metabolic reaction constitute an Operon. E.g. lac operon, trp operon, ara operon, his operon, val operon etc.
Lac Operon in E. coli
- The operon controlling lactose metabolism.
- It is proposed by Francois Jacob & Jacque Monod.
It consists of
a) A regulatory or inhibitor (i) gene: Codes for repressor protein.
b) 3 structural genes:
i. z gene: Codes for b galactosidase. It hydrolyses lactose to galactose and glucose.
ii. y gene: Codes for permease. It increases permeability of the cell to b-galactosides (lactose).
iii. a gene: Codes for a transacetylase.
- Genes in the operon function together in the same or related metabolic pathway.
- If there is no lactose (inducer), lac operon remains switched off. The regulator gene synthesizes mRNA to produce repressor protein. This protein binds to the operator region and blocks RNA polymerase movement. So the structural genes are not expressed.
- If lactose or allolactose is provided in the growth medium, it is transported into E. coli cells by the action of permease. Lactose (inducer) binds with repressor protein. So repressor protein cannot bind to operator region. The operator region becomes free and induces the RNA polymerase to bind with promoter. Then transcription starts.
- Regulation of lac operon by repressor is called negative regulation.
|In the absence of Inducer|