RNA quality control: RppH activity allowsselective removal of nonsense messages inE. coli.

Polar effect, the reduced expression level of sequencesdownstream to mutations reducing translationefficiency, is usually due to transcription termination orinefficient translation reinitiation. Untranslated mRNAsare known to be quickly degraded, probably because oftheir increased accessibility to degradative enzymes dueto the absence of translating ribosomes. In III-VI-Ioperon of phage f1, a strong polar effect is observed in agIII 5’ proximal nonsense mutant, resulting in a veryfast, RNaseE mediated, degradation of any full-lengthmRNA. RNaseE is a key component of the E. colidegradosome, the major RNA processing/degradingmachinery. Its endonucleolytic activity is stronglyenhanced by 5’-monophosphate RNAs and inhibited by5’-triphosphate ends, so that 5’-pyrophosphate removalis a key step which triggers RNA degradation. RppH, aNudix hydrolase, has been recently identified as the 5’-mRNA pyrophosphatase.We have analyzed RNA extracted from two isogenic E.coli strains, rppH+ and rppH-, infected with f1 wt andthe nonsense polar mutant. Northern hybridizationshowed identical mRNA patterns in wt and mutant RNAfrom rppH- strain. This suggestes that 5’-triphosphateprotects mRNAs against degradation, in spite of theabsence of translating ribosomes. Moreover, as somegIII mRNAs are generated by processing, thecomparation of hybridization patterns of RNAsextracted from f1 wt infected rppH+ and rppH- cells,indicated that RppH might affect primarily mRNAdegradation but not processing.