Repression of bacteriophage phi 29 early promoter C2 by viral protein p6 is due to impairment of closed complex.

Bacillus subtilis phage phi 29 encodes a very abundant protein, p6, which is a non sequence-specific DNA-binding protein. Protein p6 has the potential to bind cooperatively to the phage genome, forming a nucleoprotein complex in which the DNA adopts a right-handed toroidal conformation winding around a protein core. The formation of this complex at the right end of the phage genome where the early promoter C2 is located affects local topology, which may contribute to the promoter repression, although the underlying molecular mechanism of this repression is not presently known. In this study, we analyzed the effect of the p6 nucleoprotein complex on the formation of transcription complexes at the C2 promoter. The results obtained indicate that the nucleoprotein complex does not occlude promoter C2 to RNA polymerase because both proteins can bind to the same DNA molecule. Protein p6 binds along the fragment including the sequence adjacent to the bound polymerase, altering the structure of the transcriptional complex and affecting specifically the stability of the closed complex. The findings presented might help to answer some of the open questions about the concerted molecular mechanisms of histone-like proteins as transcriptional silencers.