E. coli RpsO mRNA decay: RNase E processing at the beginning of the coding sequence stimulates poly(A)-dependent degradation of the mRNA.

The rpsO mRNA of E. coli encoding ribosomal protein S15 is destabilized by poly(A) tails posttranscriptionally added by poly(A)polymerase I. We demonstrate here that polyadenylation also contributes to the rapid degradation of mRNA fragments generated by RNase E. It was already known that an RNase E cleavage occurring at the M2 site, ten nucleotides downstream of the coding sequence of rpsO, removes the 3' hairpin which protects the primary transcript from the attack of polynucleotide phosphorylase and RNase II. A second RNase E processing site, referred to as M3, is now identified at the beginning of the coding sequence of rpsO which contributes together with exonucleases to the degradation of messengers processed at M2. Cleavages at M2 and M3 give rise to mRNA fragments which are very rapidly degraded in wild-type cells. Poly(A)polymerase I contributes differently to the instability of these fragments. The M3-M2 internal fragment, generated by cleavages at M3 and M2, is much more sensitive to poly(A)-dependent degradation than the P1-M2 mRNA, which exhibits the same 3' end as M3-M2 but harbours the 5' end of the primary transcript. We conclude that 5' extremities modulate the poly(A)-dependent degradation of mRNA fragments and that the 5' cleavage by RNase E at M3 activates the chemical degradation of the rpsO mRNA.