Two ribosome binding sites from the gene 0.3 messenger RNA of bacteriophage T7

Ribosome-protected regions have been isolated and analyzed from the bacteriophage T7 gene 0.3 mRNA labeled in vivo. Two discrete sites which are nearly equally protected by ribosomes are obtained from what was previously assumed to be a monocistronic message. Use of appropriate T7 deletion mutant RNAs has allowed mapping of both ribosome-recognized regions. Site a is positioned very close to the 5′ terminus of the mRNA and is apparently the initiator region for the major gene 0.3 protein, which acts to overcome the host DNA restriction system. Site b is located within several hundred nucleotides of the 3′ end of the RNA and probably initiates synthesis of a small polypeptide of unknown function. Both ribosome binding sites exhibit features common to other initiator regions from Escherichia coli and bacteriophage mRNAs. The proximity of site a to the RNase III cleavage site at the left end of gene 0.3 may explain why processing by RNase III is required for efficient translation of the major gene 0.3 protein.