Mutations that remove the 2’-OH of the A residue at the 30 end of the P-site tRNA result in a 106-fold reduction in catalysis rates. This “substrate-assisted catalysis”

2’-OH of the P-site tRNA may act as part of a “proton shuttle”

TLS Online TPP Program

#Id: 6384

#Unit 2. Cellular Organization

The DNA-binding domain occupies residues 1–59. It contains two a-helices separated by a turn. This is a common DNA-binding motif known as the HTH (helix-turn-helix); the two a-helices fit into the major groove of DNA

TLS Online TPP Program

#Id: 6385

#Unit 2. Cellular Organization

Promoter mutations are also cis-acting. If they prevent RNA polymerase from binding at P-lac, the structural genes are never transcribed.

TLS Online TPP Program

#Id: 6386

#Unit 2. Cellular Organization

Mutants that cannot be expressed at all are called uninducible. Mutants that are continuously expressed are called constitutive mutants.

TLS Online TPP Program

#Id: 6387

#Unit 2. Cellular Organization

Cryptic mutants” of lac operon make b-galactosidase but still could not grow on lactose means they are essentially Y-

TLS Online TPP Program

#Id: 6388

#Unit 2. Cellular Organization

Catabolite repression is exerted through a second messenger called cyclic AMP (cAMP) and the positive regulator protein called the catabolite repressor protein (CRP), a dimer.

TLS Online TPP Program

#Id: 6389

#Unit 2. Cellular Organization

CAP–cAMP complex stimulates transcription of the lac operon by binding to an activator-binding site adjacent to the promoter and helping RNA polymerase bind to the promoter.CAP–cAMP complex stimulates transcription of the lac operon by binding to an activator-binding site adjacent to the promoter and helping RNA polymerase bind to the promoter.