Generation of bacteriophage T3 mRNAs by post-transcriptional processing by RNase III

The primary transcripts synthesized in vitro from a T3 DNA template by Escherichia coli RNA polymerase and by T3 phage-specific RNA polymerase have been characterized with regard to cleavage by RNase III and the size of the products of the cleavage reaction have been compared with those of in vivo T3 RNAs. It has been observed that the large RNA molecule synthesized in vitro by Escherichia coli RNA polymerase from the early region of T3 DNA are cleaved at specific sites by Escherichia coli RNase III to produce all the early mRNAs normally observed in T3-infected cells. In contrast, evidence presented here shows that some of the late T3 mRNAs are generated as direct products of transcription of late regions of T3 DNA by T3 RNA polymerase without mediation of RNase III, while many other late T3 mRNAs are formed by RNase III cleavage of two of the high molecular weight T3 RNA polymerase transcripts. These in vitro data appear to be in good agreement with the observed sizes of late T3 mRNAs formed in vivo in T3-infected RNase III-deficient and RNase III ⁺ Escherichia coli cells.