T7 early RNAs are generated by site-specific cleavages.

Transcription of T7 DNA by purified Escherichia coli RNA polymerase without added factors produces long RNA molecules that begin near the left end of T7 DNA and terminate at the end of the early region. An endonuclease has been isolated from uninfected E. coli that cleaves these long RNAs at five specific sites to generate RNA molecules essentially the same as the early T7 RNAs observed in vivo. This sizing factor, which may be RNase III, can act during or after RNA synthesis. Synthesis of early RNA chains has been shown to start at three strong initiators, spaced about 150-200 base-pairs apart near the left end of T7 DNA. Thus, the five cleavages by sizing factor generate the five early messenger RNAs of T7 plus three overlapping RNAs from the promoter region. RNA chains that are started at two of the strong initiators begin with A; those started at the third begin with G. A few minor initiators have also been observed, from which only short chains seem to be synthesized. Their locations in T7 DNA have not been mapped. Rho factor does not appear to be needed to generate any of these early T7 RNAs.