Search for poly(A) polymerase targets in E. coli reveals its implication in surveillance of Glu tRNA processing and degradation of stable RNAs.

Polyadenylation is a universal post-transcriptional modification involved in degradation and quality control of bacterial RNAs. In Escherichia coli, it is admitted that any accessible RNA 3' end can be tagged by a poly(A) tail for decay. However, we do not have yet an overall view of the population of polyadenylated molecules. The sampling of polyadenylated RNAs presented here demonstrates that rRNA fragments and tRNA precursors originating from the internal spacer regions of the rrn operons, in particular, rrnB are abundant poly(A) polymerase targets. Focused analysis showed that Glu tRNA precursors originating from the rrnB and rrnG transcripts exhibit long 3' trailers that are primarily removed by PNPase and to a lesser extent by RNase II and poly(A) polymerase. Moreover, 3' trimming by exoribonucleases precedes 5' end maturation by RNase P. Interestingly, characterization of RNA fragments that accumulate in a PNPase deficient strain showed that Glu tRNA precursors still harbouring the 5' leader can be degraded by a 3' to 5' quality control pathway involving poly(A) polymerase. This demonstrates that the surveillance of tRNA maturation described for a defective tRNA also applies to a wild-type tRNA.