Molecular Characterization of Three PRORP Proteins in the Moss Physcomitrella patens: Nuclear PRORP Protein Is Not Essential for Moss Viability

RNase P is a ubiquitous endonuclease that removes the 5' leader sequence from pre-tRNAs in all organisms. In Arabidopsis thaliana, RNA-free proteinaceous RNase Ps (PRORPs) seem to be enzyme(s) for pre-tRNA 5'-end processing in organelles and the nucleus and are thought to have replaced the ribonucleoprotein RNase P variant. However, the evolution and function of plant PRORPs are not fully understood. Here, we identified and characterized three PRORP-like proteins, PpPPR_63, 67, and 104, in the basal land plant, the moss Physcomitrella patens. PpPPR_63 localizes to the nucleus, while PpPPR_67 and PpPPR_104 are found in both the mitochondria and chloroplasts. The three proteins displayed pre-tRNA 5'-end processing activity in vitro. Mutants with knockout (KO) of the PpPPR_63 gene displayed growth retardation of protonemal colonies, indicating that, unlike Arabidopsis nuclear RPORPs, the moss nuclear PpPPR_63 is not essential for viability. In the KO mutant, nuclear-encoded tRNAAsp (GUC) levels were slightly decreased, whereas most nuclear-encoded tRNA levels were not altered. This indicated that most of the cytosolic mature tRNAs were produced normally without proteinaceous RNase P-like PpPPR_63. Single PpPPR_67 or 104 gene KO mutants displayed different phenotypes of protonemal growth and chloroplast tRNA(Arg) (ACG) accumulation. However, the levels of all other tRNAs were not altered in the KO mutants. In addition, in vitro RNase P assays showed that PpPPR_67 and PpPPR_104 efficiently cleaved chloroplast pre-tRNA(Arg) (CCG) and pre-tRNA(Arg) (UCU) but they cleaved pre-tRNA(Arg) (ACG) with different efficiency. This suggests that the two proteins have overlapping function but their substrate specificity is not identical.