Inactivation of the decay pathway initiated at an internal site by RNase E promotes poly(A)-dependent degradation of the rpsO mRNA in Escherichia coli.

In Escherichia coli, RNA degradation is mediated by endonucleolytic processes, frequently mediated by RNase E, and also by a poly(A)-dependent mechanism. The dominant pathway of decay of the rpsO transcripts is initiated by an RNase E cleavage occurring at a preferential site named M2. We demonstrate that mutations which prevent this cleavage slow down degradation by RNase E. All these mutations reduce the single-stranded character of nucleotides surrounding the cleavage site. Moreover, we identify two other cleavage sites which probably account for the slow RNase E-mediated degradation of the mutated mRNAs. Failure to stabilize the rpsO transcript by appending a 5' hairpin indicates that RNase E is not recruited by the 5' end of mRNA. The fact that nucleotide substitutions which prevent cleavage at M2 facilitate the poly(A)-dependent degradation of the rpsO transcripts suggest an interplay between the two mechanisms of decay. In the discussion, we speculate that a structural feature located in the vicinity of M2 could be an internal degradosome entry site promoting both RNase E cleavages and poly(A)-dependent degradation of the rpsO mRNA. We also discuss the role of poly(A)-dependent decay in mRNA metabolism.