Characterization of the transcription activator protein C1 of bacteriophage P22

We cloned, expressed, and purified the positive regulatory protein C1 of the temperate phage P22 of Salmonella typhimurium. The purified protein was characterized as to its amino acid composition, protein sequence, molecular weight, and antigenicity. P22 C1 was shown to be a tetrameric protein composed of four identical subunits with M(r) = 10,000. Moreover, we identified and characterized two P22 C1-dependent phage promoters, P(RE) and Pa23, whose function was completely dependent on C1 both in vitro and in vivo. These two promoters share a common TTGCN6TTGC/T motif in their -35 regions, the same motif recognized by the analogous phage lambda transcription activator protein cII. P22 C1 protein bound selectively to this region and centered on the TTGC repeat motif. Binding and transcription experiments demonstrated that the two promoters respond coordinately to C1 activation. Last, in contrast to lambda cII, the C1 protein exhibited little cooperativity with Escherichia coli RNA polymerase for DNA binding, but because of its stronger inherent binding ability, achieved an overall promoter affinity similar to that observed for cII at its cognate promoter signals.