N4 virion RNA polymerase sites of transcription initiation.

Coliphage N4 virion encapsulated RNA polymerase shows a marked preference for denatured N4 DNA as a template. We show that initiation on denatured N4 virion DNA occurs with in vivo specificity. The location of the in vivo and in vitro initiation sites and the corresponding DNA sequences were determined. The N4 virion RNA polymerase promoters contain extensive sequence homology from position -18 to position 1, with a conserved GC-rich heptamer centered at -12, and two sets of short inverted repeats. We suggest that the N4 virion RNA polymerase recognizes the promoter only in a novel single-stranded form, and that the formation of the initiation complex is facilitated in vivo by supercoiling and E. coli single-stranded DNA binding protein.