Question

Positive regulation of gene expression has a specific meaning relating to the effect of a regulatory protein on the gene in question. In which of the following systems does positive regulation occur?

	A. the lac repressor lac operator system
	B. catabolite repression
	C. the trp operon
	D. the lysogenic pathway of lambda.
	E. none is true

Answer 1

Ans:- option-B. Catabolite repression. Explanation given below:-

A. & B.

• The lac operon is regulated both negatively and positively.
• Negative control is brought about by the lac repressor, which is the product of lacI Gene. The operator is the site of lac repressor binding. The lac repressor is responsible for inhibiting transcription when there is no lactose.
• Positive control results from the action of CAP ( Catabolite Activator Protein). CAP functions in a global regulatory network that allows E.coli to use glucose preferentially over all other carbon and energy sources by a mechanism called catabolite repression.

C. The trp operon of E.coli is an example of a negatively controlled repressible operon. When tryptophan levels are high, tryptophan acts as a corepressor and binds the trp repressor, activating it. The trp repressor binds the operator and blocks transcription.

D. In the lysogenic pathway of lambda, negative regulation occurs. Via the lysogenic cycle, the bacteriophage's genome is not expressed and is instead integrated into the bacteria's genome to form the prophage.