RIBOSOMES INHIBIT AN RNASE-E CLEAVAGE WHICH INDUCES THE DECAY OF THE RPSO MESSENGER-RNA OF ESCHERICHIA-COLI

The hypothesis generally proposed to explain the stabilizing effect of translation on many bacterial mRNAs is that ribosomes mask endoribonu clease sites which control the mRNA decay rate. We present the first d emonstration that ribosomes interfere with a particular RNase E proces sing event responsible for mRNA decay, These experiments used an rpsO mRNA deleted of the translational operator where ribosomal protein S15 autoregulates its synthesis. We demonstrate that ribosomes inhibit th e RNase E cleavage, 10 nucleotides downstream of the rpsO coding seque nce, responsible for triggering the exonucleolytic decay of the messag e mediated by polynucleotide phosphorylase. Early termination codons a nd insertions which increase the length of ribosome-free mRNA between the UAA termination codon and this RNase E site destabilize the transl ated mRNA and facilitate RNase E cleavage, suggesting that ribosomes s terically inhibit RNase E access to the processing site. Accordingly, a mutation which reduces the distance between these two sites stabiliz es the mRNA, Moreover, an experiment showing that a 10 nucleotide inse rtion which destabilizes the untranslated mRNA does not affect mRNA st ability when it is inserted in the coding sequence of a translated mRN A demonstrates that ribosomes can mask an RNA feature, 10-20 nucleotid es upstream of the processing site, which contributes to the RNase E c leavage efficiency.