Specific sequences and a hairpin structure in the template strand are required for N4 virion RNA polymerase promoter recognition.

Coliphage N4 virion-encapsidated, DNA-dependent RNA polymerase (vRNAP) is inactive on double-stranded N4 DNA; however, denatured promoter-containing templates are accurately transcribed. We report that all determinants of vRNAP promoter recognition exist in the template strand, indicating that this enzyme is a site-specific, single-stranded DNA-binding protein. We show that conserved sequences and the integrity of inverted repeats present at the promoters are essential for activity, suggesting the necessity for specific secondary structure. Evidence for such a structure is presented. We propose a model for in vivo utilization of vRNAP promoters in which template negative supercoiling yields single-strandedness at the promoter to reveal the determinants of vRNAP binding. This structure is stabilized by the binding of E. coli single-stranded DNA-binding protein to yield an "activated promoter."