Clockwise rotation gives dihedral angles from 0º to 180º.

180º is the same as counter-clockwise rotation to -180º.

#Id: 576

#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

#Id: 577

#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.

#Id: 578

#Unit 2. Cellular Organization

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

#Id: 579

#Unit 2. Cellular Organization

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

#Id: 580

#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.

#Id: 581

#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.