Escherichia coli RNA polymerase and trp repressor interaction with the promoter-operator region of the tryptophan operon of Salmonella typhimurium

Abstract

Close contacts between Escherichia coli RNA polymerase and specific purine residues in the tryptophan ( trp) operon promoter of Salmonella typhimurium were revealed using the methylating agent dimethyl sulfate. RNA polymerase bound to trp promoter DNA caused alterations in the rate of methylation at seven specific sites; in the anti-sense strand, guanine residues at positions −37, −34 and −2 showed enhanced methylation, while those at positions −14, −6 and +3 showed reduced methylation. In the sense strand, only the guanine residue at −32 showed reduced methylation. No RNA polymerase contacts with adenine residues were observed. Using the same method, close interactions between E. coli trp repressor and purine residues in the trp operator of S. typhimurium were examined. Bound trp repressor alters the methylation rates of both guanine and adenine residues from positions −25 to +3. The points of contact are distributed rather symmetrically on both DNA strands. Three points of close contact are shared by RNA polymerase and trp repressor, supporting previous models of trp repressor action.