Homologies and divergences in the transcription regulatory system of two related Bacillus subtilis phages.

Transcription regulation relies on the molecular interplay between the RNA polymerase and regulatory factors. Phages of the phi29-like genus encode two regulatory proteins, p4 and p6. In phi29, the switch from early to late transcription is based on the synergistic binding of proteins p4 and p6 to the promoter sequence, resulting in a nucleosome-like structure able to synergize or antagonize the binding of RNAP. We show that a nucleosome-like structure of p4 and p6 is also formed in the related phage Nf and that this structure is responsible for the coordinated control of the early and late promoters. However, in spite of their homologies, the transcriptional regulators are not interchangeable, and only when all of the components of the Nf regulatory system are present is fully active transcriptional regulation of the Nf promoters achieved.