Functional interactions between a phage histone-like protein and a transcriptional factor in regulation of φ29 early-late transcriptional switch

Protein pb is a nonspecific DNA-binding protein occurring in high abundance in phage phi 29-infected cells. Here, we demonstrate a novel role for this versatile histone-like protein: its involvement in regulating the viral switch between early and late transcription. pb performs this role by exhibiting a reciprocal functional interaction with the regulatory protein p4, also phage encoded, which is required for repression of the early A2b and A2c promoters and activation of the late A3 promoter. On the one hand, p6 promotes p4-mediated repression of the A2b promoter and activation of the A3 promoter by enhancing binding of p4 to its recognition site at PA3; on the other, p4 promotes p6-mediated repression of the A2c promoter by favoring the formation of a stable p6-nucleoprotein complex that interferes with RNA polymerase binding to PA2c. We propose that the observed interplay between proteins p6 and p4 is based on their DNA architectural properties.