Mapping the cAMP receptor protein contact site on the α subunit of Escherichia coli RNA polymerase

The C‐terminal region (amino acid residues 236–329) of the Escherichia coli RNA polymerase α subunit carries the contact site I for positive transcription factors. For detailed mapping of the contact site for the cAMP receptor protein (CRP), we made a library of mutant rpoA by polymerase chain reaction (PCR) mutagenesis, such that each should carry a single mutation on average and exclusively in the C‐terminal half of the rpoA gene, and then screened this library for mutants with decreased expression of the lacZ gene. Reconstituted holoenzyme containing the mutant a subunits transcribed galP1 but not lacP1 in vitro in the presence of cAMP–CRP. DNA sequence determination of several ‘Lac^‐’ mutant rpoA genes revealed that all had mutations clustered within a short segment near the C‐terminus of α between amino acid residues 265 and 270. A cluster of contact sites appear to exist within the contact site I region, each comprising of about five amino acids and responding in molecular communication with a different transcription factor(s).