Catabolite Activator protein in Lac operon is a -
Ans. is 'a' i.e., Positive regulator* The lac operon is a region of DNA in the genome of E. coli that contains following genetic elements -i) Three structural genes# These code for 3 proteins that are involved in catabolism of lactose. These genes are 'Z' gene (codes for b-galactosidase), 'g' gene (codes for galactoside permease), and 'A' gene (codes for thiogalactoside transacetylase).ii) Regulatory gene (lac i)# It produces repressor protein.iii) A promotor site (P)# It is the binding site for RNA polymerase. It contains two specific regions -a) CAP site (Catabolite activator protein binding site).b) RNA polymerase binding siteiv) An operator site (O)"-# Repressor binds to this site and blocks transcription.* 3 Structural genes are expressed only when 'O' site is empty (repressor is not bound) and the CAP site is bound by a complex of cAMP and CAP (catabolite gene activator protein).* 3 situations are possible -1) When glucose is the only sugar available-# In this case, the lac operon is repressed (turned off).# Repression is mediated by the repressor protein binding via helix-turn-helix motif to the operator site. Binding interferes with the progress of RNA polymerase, and blocks transcription of the structural genes.# This is an example of negative regulation.2) When only lactose is available (No glucose).# In this case, the lac operon is induced (turned on).# A small amount of lactose is converted to an isomer, allolactose. This compound is an inducer that binds to the repressor protein, changing its configuration so that it can no longer bind to the operator.# In absence of glucose, adenylyl cyclase is active and sufficient quantitive of cAMP are made and bind to the CAP-binding site, causing RNA polymerase to more efficiently initiate transcription at the promoter site.# This is an eg. of positive regulation3) When both glucose and lactose are available -# In this case transcription of lac operon is negligible, even if lactose is present at a higher concentration.# Adenylyl cyclase is deactivated in the presence of glucose-a process known as catabolite repression, So no cAMP-CAP complex forms and the CAP binding site remains empty.# RNA polymerase is unable to effectively initiate the transcription, even though the repressor may not be bound to the operator region.
Positive regulator
Promoter
Repressor
Negative regulator
Molecular Genetics