MUTANTS WITH SUBSTITUTIONS FOR GLU171 IN THE CATABOLITE ACTIVATOR PROTEIN (CAP) OF ESCHERICHIA-COLI ACTIVATE TRANSCRIPTION FROM THE LAC PROMOTER

Single amino acid substitutions for residue Glu171 in helix E of the c atabolite gene activator protein (CAP) of Escherichia coli have been r eported to abolish activation of transcription without impairing bindi ng to the CAP site of the lac promoter. The negative charge of Glu171 was proposed to transmit the activating signal from CAP to RNA polymer ase. However, this idea has been challenged by later work. We set up a system to re-examine this issue. We analysed the ability of mutant CA P-E171L and CAP-E171K proteins to bind a near-consensus CAP site in vi vo and found it to be diminished fourfold relative to wild type in eac h case. Activation of lac transcription by these mutant proteins remai ns the same as with wild-type CAP. Thus our results confirm that Glu17 1 in helix E of CAP is not involved directly in the activation of tran scription. Yet CAP-E171K does not activate transcription as well as wi ld-type CAP under all circumstances. Possible reasons for this absence of activation are discussed.