T7 early messenger RNAs are the direct products of ribonuclease III cleavage

T7 early RNAs were synthesized in vitro by transcribing T7 DNA with Escherichia coli RNA polymerase and treating the resulting precursor molecule with ribonuelease III. Oligonucleotide fragments from the 5′ and 3′ termini of several of the cleaved species were then selectively isolated. Structural analysis revealed sequences identical to the corresponding in vivo RNAs. Thus, the T7 early RNAs found in phage-infected cells appear to be the direct products of RNAase III cleavage of a large precursor molecule. We conclude further that RNAase III action on this particular natural substrate is a sequence-specific event.