Frpo: a novel single-stranded DNA promoter for transcription and for primer RNA synthesis of DNA replication.

We describe a novel promoter for E. coli RNA polymerase that functions efficiently only in the form of single-stranded DNA. Derived from the leading region of F plasmid, single-stranded Frpo sequence directs RNA polymerase to initiate transcription at a specific site within Frpo, and this specific transcription is highly stimulated by SSB. Prior denaturation activates transcription from otherwise inactive duplex DNA containing Frpo. Since RNAs synthesized on SSB-coated single-stranded Frpo are efficiently elongated into DNA chains by DNA polymerase III holoenzyme, transcription at Frpo serves also for priming DNA replication. A mode of recognition by RNA polymerase of a unique secondary structure within Frpo is proposed, and possible roles of this novel single-stranded promoter in expression and replication during conjugal transfer of F plasmid are discussed.