Involvement of the Escherichia coli endoribonucleases G and E in the secondary processing of RegB-cleaved transcripts of bacteriophage T4

Sequence-specific endoribonuclease RegB of bacteriophage T4 cleaves early phage mRNAs and facilitates the transition between early and subsequent phases of T4 gene expression. The great majority of RegB targets have been identified in the intergenic regions of T4 transcripts, frequently in the Shine-Dalgarno sequences. Here we show that localization of RegB targets is not restricted to intergenic regions of mRNA. We detected 30 intragenic RegB sites in T4 transcripts that are differently susceptible to cleavage. Four RegB-processed mRNAs were previously shown to undergo further processing at so-called “secondary sites”. We have found three additional transcripts carrying clear targets for both RegB and another endoribonuclease. We show that secondary cuts within RegB-processed T4 mRNAs are generated mainly by Escherichia coli RNase G, but that in some cases RNase E can recognize the same targets. Using plasmid-phage systems we demonstrate that T4 infection favours cleavage by the host endoribonucleases at these sites.