The bacteriophage N4-coded single-stranded DNA-binding protein (N4SSB) is the transcriptional activator of Escherichia coli RNA polymerase at N4 late promoters.

Transcription of the 72kb linear double-stranded DNA genome of coliphage N4 is carried out by the sequential activity of three different RNA polymerases. Early and middle viral transcripts are synthesized by two phage-coded RNA polymerases while late transcription is carried out by the Escherichia coli sigma 70-RNA polymerase. We have determined the sequences and sites of initiation of several N4 late transcripts; N4 late promoters share weak homology with the E. coli sigma 70 promoter consensus sequence. Indeed, N4 late promoters are weak templates for the host enzyme. We present evidence that the phage-coded, single-stranded DNA-binding protein (N4SSB), a protein that is required for phage DNA replication and recombination and does not bind with sequence specificity to DNA, is the activator of E. coli RNA polymerase at late N4 promoters. Models for the mechanism of action of N4SSB as a transcriptional activator are discussed.

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