Protein-only RNase P function in Escherichia coli: viability, processing defects and differences between PRORP isoenzymes

The RNase P family comprises structurally diverse endoribonucleases ranging from complex ribonucleoproteins to single polypeptides. We show that the organellar (AtPRORP1) and the two nuclear (AtPRORP2,3) single-polypeptide RNase P isoenzymes from Arabidopsis thaliana confer viability to Escherichia coli cells with a lethal knockdown of its endogenous RNA-based RNase P. RNA-Seq revealed that AtPRORP1, compared with bacterial RNase P or AtPRORP3, cleaves several precursor tRNAs (pre-tRNAs) aberrantly in E. coli. Aberrant cleavage by AtPRORP1 was mainly observed for pre-tRNAs that can form short acceptor-stem extensions involving G:C base pairs, including tRNAAsp(GUC), tRNASer(CGA) and tRNAHis. However, both AtPRORP1 and 3 were defective in processing of E. coli pre-tRNASec carrying an acceptor stem expanded by three G:C base pairs. Instead, pre-tRNASec was degraded, suggesting that tRNASec is dispensable for E. coli under laboratory conditions. AtPRORP1, 2 and 3 are also essentially unable to process the primary transcript of 4.5S RNA, a hairpin-like non-tRNA substrate processed by E. coli RNase P, indicating that PRORP enzymes have a narrower, more tRNA-centric substrate spectrum than bacterial RNA-based RNase P enzymes. The cells' viability also suggests that the essential function of the signal recognition particle can be maintained with a 5΄-extended 4.5S RNA.