Explain the negative control of transcription initiation as described by Jacob and Monod.

Explanation

(i) Jacob and Monod proposed the classical model of Lac operon to explain gene expression and regulation in E.coli. In lac operon, a polycistronic structural gene is regulated by a common promoter and regulatory gene.
(ii) When the cell is using its normal energy source as glucose, the .i. gene transcribes a repressor mRNA and after its translation, a repressor protein is produced. It binds to the operator region of the operon and prevents translation, as a result, `beta`-galactosidase is not produced.
(iii) In the absence of preferred carbon source such as glucose, if lactose is available as an energy source for the bacteria then lactose enters the cell as a result of permease enzyme. Lactose acts as an inducer and interacts with the repressor to inactivate it.
(iv) The repressor protein binds to the operator of the operon and prevents RNA polymerase from transcribing the operon.
(v) In the presence of inducer, such as lactose, or allolactose, the repressor is inactivated by interaction with the inducer. This allows RNA polymerase to bind to the promoter site and transcribe the operon to produce lac mRNA which enables formation of all the required enzymes needed for lactose metabolism.
(vi) This regulation of lac operon by the repressor is an example of negative control of transcription initiation.