Cleavages in the 5' region of the ompA and bla mRNA control stability: studies with an E. coli mutant altering mRNA stability and a novel endoribonuclease.

We describe here the partial purification of a novel Escherichia coli endoribonuclease, RNase K. This protein catalyses site-specific cleavages in the 5' region of in vitro transcribed ompA and bla transcripts. Some of the resulting cleavage products are also found in cellular ompA mRNA, defining the in vivo activity of RNase K. The following evidence suggests that RNase K initiates mRNA degradation. First, RNase K cleavages are suppressed in the ams mutant, which has a generally prolonged mRNA half-life. Secondly, RNase K cleavage products seem to have very short half-lives in vivo, indicating that they are decay intermediates rather than processing products. Thirdly, the differences in in vivo half-life between the ompA and bla mRNAs are mimicked in in vitro decay reactions with purified RNase K. The relationship between RNase K and the ams locus might point to a more general role of RNase K in mRNA degradation. We discuss the influence of mRNA secondary structure on RNase K cleavage specificity.