Processing of mRNA by ribonuclease III regulates expression of gene 1.2 of bacteriophage T7.

A bacteriophage T7 mutation, HS9, is phenotypically defective in gene 1.2, although it maps outside the gene. The single nucleotide change responsible for the HS9 mutation lies within the RNAase III recognition site immediately following gene 1.2. This RNAase III recognition site, responsible for the processing of the mRNA encoding genes 1.1 and 1.2, contains two cleavage sites, separated by 29 bases. The HS9 mutation prevents cutting by RNAase III at one site in vitro, yielding a mRNA containing an additional 29 bases at its 3' end. The ten second-site reversion mutations of HS9 are all located in the RNAase III recognition site and either restore or eliminate cutting at both sites. RNAase III mutants of Escherichia coli phenotypically suppress the HS9 mutation. We propose that the extra 29 bases at the 3' end of the mRNA hybridize to the ribosome-binding site of gene 1.1; gene 1.1 immediately precedes gene 1.2 on the same mRNA molecule. Such hybridization prevents the initiation of translation of this mRNA containing gene 1.1. A strong polar effect represses the translation of gene 1.2.