Transcription from the late promoters of bacteriophage P2 and its satellite phage P4 is activated by a unique class of small, zinc-binding proteins, Using plasmid expression systems, we compared activators from two P2-like (helper) phages with those encoded by two satellite phages, The helper phage activators have more activity on the P4 phage sid promoter, In contrast, the satellite phage activators function better on the four late PZ promoters and on the P4 late leftward promoter, We purified one activator encoded by a P2-like phage and an activator from a satellite phage and determined their binding sites within the P2 and P4 late promoters. Differences in activity levels correlate with binding specificities; promoters that function best with the satellite phage activators have only one activator binding site centered at -55, while the P4 sid promoter, which has more activity with helper phage activators, has a second binding site centered at -18, Surprisingly, DNase I footprinting revealed only very minor differences in promoter binding by the two activators reported here and the P4 activator reported previously, Thus, the differences in transcriptional activity are probably due to interactions between the activators and RNA polymerase, rather than interactions between the activators and DNA.
