The virus-specified subunits of a modified B. subtilis RNA polymerase are determinants of DNA binding and RNA chain initiation.

The phage SPO1-modified RNA polymerase B-P can form rapidly initiating complexes with SPO1 DNA but not with heterologous phi1 DNA. The B-P enzyme binds only weakly to heterologous phi29 DNA: preincubation with phi29 DNA does not substantially slow the formation of rapidly initiating complexes between polymerase B-P and subsequently added SPO1 DNA. In contrast, B. subtilis holoenzyme and core polymerase are substantially sequestered by preincubation with phi29 DNA. The results show that at least one of the phage SPO1-coded subunits of the polymerase B-P determines selective transcription at the level of DNA binding and RNA chain initiation, weakens the binding of RNA polymerase core to heterologous DNA, and discriminates against promoter complex formation at certain promoters that are utilized by the B. subtilis holoenzyme.