The switch from early to late transcription in phage GA-1: Characterization of the regulatory protein p4(G)

The transcription program of the Bacillus phage GA-1, a distant relative of phage Phi 29, has been studied. Transcription of the GA-1 genome occurred in two stages, early and late. Early genes were expressed from two promoter s equivalent to the Phi 29 A2b and A2c promoters, whereas late transcriptio n started at a site equivalent to the Phi 29 late A3 promoter. The activity of the GA-1 early A2b and A2c promoters diminished 10 minutes after infect ion, a time at which expression of the late promoter increased significantl y. The switch from early to late transcription required protein synthesis, suggesting the need for viral protein(s). An open reading frame was found i n the GA-1 genome coding for a protein showing a 53% similarity to Phi 29 r egulatory protein p4, and was named p4(G). In Phi 29, protein p4 represses the early A2b and A2c promoters and activates the late A3 promoter by recru iting RNA polymerase to it. A binding site for protein p4, was localized up stream from the GA-1 late A3 promoter, overlapping with the early A2b promo ter. In vitro, protein p4, prevented the binding of RNA polymerase to the G A-1 early A2b promoter but, unlike in Phi 29, had no effect on the expressi on of the late A3 promoter: RNA polymerase could efficiently bind and initi ate transcription from the A3 promoter in the absence of protein p4G. There fore, activation of late transcription occurs differently in GA-1 and Phi 2 9. We propose that protein p4G is an anti-repressor which inhibits the bind ing to the late promoter of an unknown repressor factor present in the host strain. (C) 1999 Academic Press.