RNA quality control: RppH activity allows selective removal of nonsense messages in E. coli.

Polar effect, the reduced expression level of sequences downstream to mutations reducing translation efficiency, is usually due to transcription termination or inefficient translation reinitiation. Untranslated mRNAs are known to be quickly degraded, probably because of their increased accessibility to degradative enzymes due to the absence of translating ribosomes. In III-VI-I operon of phage f1, a strong polar effect is observed in a gIII 5’ proximal nonsense mutant, resulting in a very fast, RNaseE mediated, degradation of any full-length mRNA. RNaseE is a key component of the E. coli degradosome, the major RNA processing/degrading machinery. Its endonucleolytic activity is strongly enhanced by 5’-monophosphate RNAs and inhibited by 5’-triphosphate ends, so that 5’-pyrophosphate removal is a key step which triggers RNA degradation. RppH, a Nudix 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 and the nonsense polar mutant. Northern hybridization showed identical mRNA patterns in wt and mutant RNA from rppH- strain. This suggestes that 5’-triphosphate protects mRNAs against degradation, in spite of the absence of translating ribosomes. Moreover, as some gIII mRNAs are generated by processing, the comparation of hybridization patterns of RNAs extracted from f1 wt infected rppH+ and rppH- cells, indicated that RppH might affect primarily mRNA degradation but not processing.