Asymmetric arrangement of two alpha subunits within Escherichia coli RNA polymerase : involvement of one alpha subunit in contact with cAMP receptor protein

Class I transcription factors of Escherichia coli have been proposed to make contact with contact site I on the α subunit, C-terminal region of RNA polymerase with the subunit composition of α2β β′σ. Both a reconstituted mutant holoenzyme containing two C-terminally truncated α-235 subunits and a hybrid enzyme containing one wild-type α(α-329) and one C-terminal truncated α(α-235) subunit were found to be as active in transcription from factor-independent simple promoters as the wild-type holoenzyme. The mutant enzyme was, however, inactive in cAMP receptor protein (CRP)-dependent transcription from lacP1 promoter, but the hybrid enzyme was about 50% as active in larPI transcription as the wild-type enzyme. The results indicate that only one specific α subunit makes contact with CRP.