MULTIPLE DEGRADATION PATHWAYS OF THE RPSO MESSENGER-RNA OF ESCHERICHIA-COLI - RNASE-E INTERACTS WITH THE 5'-EXTREMITIES AND 3'-EXTREMITIES OF THE PRIMARY TRANSCRIPT

The degradation process of the rpsO mRNA is one of the best characteri sed in E coli. Two independent degradation pathways have been identifi ed. The first one is initiated by an RNase E endonucleolytic cleavage which allows access to the transcript by polynucleotide phosphorylase and RNase II. Cleavage by RNase E gives rise to an rpsO message lackin g the stabilising hairpin of the primary transcript; this truncated mR NA is then degraded exonucleolytically from its 3' terminus. This path way might be coupled to the translation of the message. The second pat hway allows degradation of polyadenylated rpsO mRNA independently of R Nase II, PNPase and RNase E. The ribonucleases responsible for degrada tion of poly(A) mRNAs under these conditions are not known. Poly(A) ta ils have been proposed to facilitate the degradation of structured RNA by polynucleotide phosphorylase. In contrast, we believe that removal of poly(A) by RNase II stabilises the rpsO mRNA harbouring a 3' hairp in. In addition to these two pathways, we have identified endonucleoly tic cleavages which occur only in strains deficient for both RNase E a nd RNase III suggesting that these two endonucleases protect the 5' le ader of the mRNA from the attack of unidentified ribonuclease(s). Loop ing of the rpsO mRNA might explain how RNase E bound at the 5' end can cleave at a site located just upstream the hairpin of the transcripti on terminator.