In vitro requirements for N4 RNA polymerase II-specific initiation.

The synthesis of coliphage N4 middle RNAs requires the activity of three phage-coded proteins, P7, P4, and P17, of molecular weights 40,000, 30,000, and 15,000, respectively. P4 and P7 constitute a heterodimeric RNA polymerase, N4 RNA polymerase II, which cannot transcribe or bind to native N4 DNA. Denatured templates are transcribed but without specificity. We present the development of an in vitro transcription system, composed of purified N4 RNA polymerase II and solubilized P17, which accurately utilizes in vivo sites of transcription initiation. Possible mechanisms for P17 action are discussed.