Question

Which has the highest Ka for the site GAATTC?

-LacI

-EcoR1

-Cro

-Cl

-IHF

Answer 1

Ka is the association constant, which is inverse of dissociation constant. Ka determines the affinity of the molecules for the site. Higher the Ka, more is the affinity of enzyme or molecule for the site. GAATTC is the palindromic sequence that is recognized and cleaved exclusively by the restriction enzyme ECOR1. A change in single base pair in GAATTC will decrease the binding of ECOR1 by a large extent. ECOR1 cleaves this site to form sticky ends a sit cleaves between G and A.

LacI is the gene that codes for the lac repressor. This lac repressor binds to operator of lac operon. Lac repressor does not bind to GAATTC. Cl and Cro are the repressor in bacteriophage lambda. CI protein binds to OR1 and OR2 operators and blocks the transcription of Cro repressor, to maintain the lysogenic cycle. Cro repressor is required for lytic cycle by binding to OR3 operator. IHF is integration host factor that binds to UAS1 sequence on promoter. It activates the ilvG-MEDA operon present in E .coli.

Right Choice: ECOR1