Molecular interplay between RNA polymerase and two transcriptional regulators in promoter switch.

Transcription regulation relies in the molecular interplay between the RNA polymerase (RNAP) and regulatory factors. Phage phi29 promoters A2c, A2b and A3 are coordinately regulated by the transcriptional regulator protein p4 and the histone-like protein p6. This study shows that protein p4 binds simultaneously to four sites: sites 1 and 2 located between promoters A2c and A2b and sites 3 and 4 between promoters A2b and A3, placed in such a way that bound p4 is equidistant from promoters A2c and A2b and one helix turn further upstream from promoter A3. The p4 molecules bound to sites 1 and 3 reorganise the binding of protein p6, giving rise to the nucleoprotein complex responsible for the switch from early to late transcription. We identify the positioning of the alphaCTD-RNAP domain at these promoters, and demonstrate that the domains are crucial for promoter A2b recognition and required for full activity of promoter A2c. Since binding of RNAP overlaps with p4 and p6 binding, repression of the early transcription relies on the synergy of the regulators able to antagonize the stable binding of the RNAP through competition for the same target, while activation of late transcription is carried out through the stabilization of the RNAP by the p4/p6 nucleoprotein complex. The control of promoters A2c and A2b by feed-back regulation is discussed.