MOLECULAR GENETICS 12

 Regulation of gene expression

Gene expression results in the formation of a polypeptide and it can be regulated at several levels such as

• transcriptional level (formation of primary transcript),
• processing level (regulation of splicing),
• transport of mRNA from nucleus to the cytoplasm,
• translational level.

 In prokaryotes, control of the rate of transcriptional initiation is the predominant site for control of gene expression.

In a transcription unit, the activity of RNA polymerase at a given promoter is in turn regulated by interaction with accessory proteins which can act both positively (activators) and negatively (repressors). 

Regulation of gene expression can be studied with the help of Lac operon.

Lac operon

• Lac refers to lactose in lac operon.

• The lac operon consists of one regulatory gene the i gene which codes for the repressor of the lac operon and three structural genes (z, y, and a)

• The z gene codes for beta-galactosidase (β-gal), which hydrolyses disaccharide, lactose into galactose and glucose.
• The y gene codes for permease, which increases permeability of the cell to β-galactosides.
• The a gene encodes a transacetylase.

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• Lactose is termed as inducer as lactose is the substrate for the enzyme beta-galactosidase and it regulates switching on and off of the operon.

• In the absence of inducer
• The repressor of the operon is synthesized (all-the-time – constitutively) from the i gene.
• The repressor protein binds to the operator region of the operon and prevents RNA polymerase from transcribing the operon.

• In the presence of inducer
• The repressor is inactivated by interaction with the inducer which allows RNA polymerase access to the promoter and transcription proceeds.

• Regulation of lac operon by repressor is referred to as negative regulation.

Positive regulation

Tryptophan operon