These authors contributed equally to the manuscript. SUMMARYThe pentatricopeptide repeat (PPR) proteins form one of the largest protein families in land plants. They are characterised by tandem 30-40 amino acid motifs that form an extended binding surface capable of sequence-specific recognition of RNA strands. Almost all of them are post-translationally targeted to plastids and mitochondria, where they play important roles in post-transcriptional processes including splicing, RNA editing and the initiation of translation. A code describing how PPR proteins recognise their RNA targets promises to accelerate research on these proteins, but making use of this code requires accurate definition and annotation of all of the various nucleotide-binding motifs in each protein. We have used a structural modelling approach to define 10 different variants of the PPR motif found in plant proteins, in addition to the putative deaminase motif that is found at the C-terminus of many RNA-editing factors. We show that the super-helical RNA-binding surface of RNA-editing factors is potentially longer than previously recognised. We used the redefined motifs to develop accurate and consistent annotations of PPR sequences from 109 genomes. We report a high error rate in PPR gene models in many public plant proteomes, due to gene fusions and insertions of spurious introns. These consistently annotated datasets across a wide range of species are valuable resources for future comparative genomics studies, and an essential pre-requisite for accurate large-scale computational predictions of PPR targets. We have created a web portal (http://www.-plantppr.com) that provides open access to these resources for the community.

The ubiquitous endonuclease RNase P is responsible for the 5' maturation of tRNA precursors. Until the discovery of human mitochondrial RNase P, these enzymes had typically been found to be ribonucleoproteins, the catalytic activity of which is associated with the RNA component. Here we show that, in Arabidopsis thaliana mitochondria and plastids, a single protein called 'proteinaceous RNase P' (PRORP1) can perform the endonucleolytic maturation of tRNA precursors that defines RNase P activity. In addition, PRORP1 is able to cleave tRNA-like structures involved in the maturation of plant mitochondrial mRNAs. Finally, we show that Arabidopsis PRORP1 can replace the bacterial ribonucleoprotein RNase P in Escherichia coli cells. PRORP2 and PRORP3, two paralogs of PRORP1, are both localized in the nucleus.

RNase P is an essential enzyme that cleaves the 59 leader sequence of tRNA precursors. RNase Ps were believed until now to occur universally as ribonucleoproteins in organisms performing RNase P activity. Here we find that protein-only RNase P enzymes called PRORP (for proteinaceous RNase P) support RNase P activity in vivo in both organelles and the nucleus in Arabidopsis. Beyond tRNA, PRORP proteins are involved in the maturation of small nucleolar RNA (snoRNA) and mRNA. Finally, ribonucleoprotein RNase MRP is not involved in tRNA maturation in plants. Altogether, our results indicate that ribonucleoprotein enzymes have been entirely replaced by proteins for RNase P activity in plants.Supplemental material is available for this article.Received February 10, 2012; revised version accepted April 5, 2012.RNase P is a virtually universal enzyme involved in the maturation of tRNAs, as it cleaves the 59 leader sequence of tRNA precursors. It is thus essential to obtain functional tRNAs and is therefore pivotal for translation (Lai et al. 2010;Reiter et al. 2010). RNase P activities from all phyla of life were assumed to be universally performed by ribonucleoprotein enzymes whose catalytic activities are held by ribozymes (Altman 2007). This concept was first challenged with the proposition that spinach chloroplast and human mitochondria RNase Ps would not contain any RNA moiety (Wang et al. 1988;Rossmanith and Karwan 1998). More recently, protein-only RNase P enzymes called PRORP (for proteinaceous RNase P) have been characterized at the molecular level in endosymbiotic organelles in both humans and Arabidopsis (Holzmann et al. 2008;Gobert et al. 2010). Still, the dogma remained that RNase P enzymes would nonetheless universally occur as ribonucleoproteins in living organisms performing RNase P activity, with protein-only RNase Ps being marginal exceptions restricted to only some organelles (e.g., Esakova and Krasilnikov 2010).The putative PRORP RNase P enzymes are characterized by the occurrence of a conserved ''NYN'' metallonuclease domain (Anantharaman and Aravind 2006). PRORP proteins also belong to the huge pentatricopeptide repeat (PPR) protein family. These proteins, typically from eukaryotes, are involved in a wide variety of posttranscriptional mechanisms (Schmitz-Linneweber and Small 2008). However, no functional information was available for PRORP proteins. We previously established that Arabidopsis PRORP1, a protein localized in organelles, can act in vitro as an RNase P enzyme that is a single protein (Gobert et al. 2010), although its function remained elusive in planta. We also used localization experiments (YFP fusions and immunodetections) to determine that PRORP2 and PRORP3, two paralogs of PRORP1, were present in Arabidopsis nuclei (Gobert et al. 2010). In addition, the fast-growing amount of genomic data has revealed that some important groups of eukaryotes, such as land plants and kinetoplastids, do not encode any recognizable genes for RNase P RNA or for proteins specific for ribonucleoprotein...

RNase P is the essential activity removing 5′-leader sequences from transfer RNA precursors. RNase P was always associated with ribonucleoprotein complexes before the discovery of protein-only RNase P enzymes called PRORPs (PROteinaceous RNase P) in eukaryotes. Here we provide biophysical and functional data to understand the mode of action of PRORP enzymes. Activity assays and footprinting experiments show that the anticodon domain of transfer RNA is dispensable, whereas individual residues in D and TψC loops are essential for PRORP function. PRORP proteins are characterized in solution and a molecular envelope is derived from small-angle X-ray scattering. Conserved residues are shown to be involved in the binding of one zinc atom to PRORP. These results facilitate the elaboration of a model of the PRORP/transfer RNA interaction. The comparison with the ribonucleoprotein RNase P/transfer RNA complex suggests that transfer RNA recognition by PRORP proteins is similar to that by ribonucleoprotein RNase P.