Archaeal-bacterial chimeric RNase P RNAs: towards understanding RNA's architecture, function and evolution

Multiple structural flavors of RNase P in precursor tRNA processing

The precursor transfer RNAs (pre-tRNAs) require extensive processing to generate mature tRNAs possessing proper fold, structural stability, and functionality required to sustain cellular viability. The road to tRNA maturation follows an ordered process: 5'-processing, 3'-processing, modifications at specific sites, if any, and 3'-CCA addition before aminoacylation and recruitment to the cellular protein synthesis machinery. Ribonuclease P (RNase P) is a universally conserved endonuclease in all domains of life, performing the hydrolysis of pre-tRNA sequences at the 5' end by the removal of phosphodiester linkages between nucleotides at position -1 and +1. Except for an archaeal species: Nanoarchaeum equitans where tRNAs are transcribed from leaderless-position +1, RNase P is indispensable for life and displays fundamental variations in terms of enzyme subunit composition, mechanism of substrate recognition and active site architecture, utilizing in all cases a two metal ion-mediated conserved catalytic reaction. While the canonical RNA-based ribonucleoprotein RNase P has been well-known to occur in bacteria, archaea, and eukaryotes, the occurrence of RNA-free protein-only RNase P in eukaryotes and RNA-free homologs of Aquifex RNase P in prokaryotes has been discovered more recently. This review aims to provide a comprehensive overview of structural diversity displayed by various RNA-based and RNA-free RNase P holoenzymes towards harnessing critical RNA-protein and protein-protein interactions in achieving conserved pre-tRNA processing functionality. Furthermore, alternate roles and functional interchangeability of RNase P are discussed in the context of its employability in several clinical and biotechnological applications. This article is categorized under: RNA Processing > tRNA Processing RNA Evolution and Genomics > RNA and Ribonucleoprotein Evolution RNA Interactions with Proteins and Other Molecules > RNA-Protein Complexes.

Characterization of RNA-based and protein-only RNases P from bacteria encoding both enzyme types

A small group of bacteria encode two types of RNase P, the classical ribonucleoprotein (RNP) RNase P as well as the protein-only RNase P HARP (homolog of Aquifex RNase P). We characterized the dual RNase P activities of five bacteria that belong to three different phyla. All five bacterial species encode functional RNA (gene rnpB) and protein (gene rnpA) subunits of RNP RNase P, but only the HARP of the thermophile Thermodesulfatator indicus (phylum Thermodesulfobacteria) was found to have robust tRNA 5'-end maturation activity in vitro and in vivo in an Escherichia coli RNase P depletion strain. These findings suggest that both types of RNase P are able to contribute to the essential tRNA 5'-end maturation activity in T. indicus, thus resembling the predicted evolutionary transition state in the progenitor of the Aquificaceae before the loss of rnpA and rnpB genes in this family of bacteria. Remarkably, T. indicus RNase P RNA is transcribed with a P12 expansion segment that is posttranscriptionally excised in vivo, such that the major fraction of the RNA is fragmented and thereby truncated by ∼70 nt in the native T. indicus host as well as in the E. coli complementation strain. Replacing the native P12 element of T. indicus RNase P RNA with the short P12 helix of Thermotoga maritima RNase P RNA abolished fragmentation, but simultaneously impaired complementation efficiency in E. coli cells, suggesting that intracellular fragmentation and truncation of T. indicus RNase P RNA may be beneficial to RNA folding and/or enzymatic activity.

Rational Design of an Orthogonal Pair of Bimolecular RNase P Ribozymes through Heterologous Assembly of Their Modular Domains

The modular structural domains of multidomain RNA enzymes can often be dissected into separate domain RNAs and their noncovalent assembly can often reconstitute active enzymes. These properties are important to understand their basic characteristics and are useful for their application to RNA-based nanostructures. Bimolecular forms of bacterial RNase P ribozymes consisting of S-domain and C-domain RNAs are attractive as platforms for catalytic RNA nanostructures, but their S-domain/C-domain assembly was not optimized for this purpose. Through analysis and engineering of bimolecular forms of the two bacterial RNase P ribozymes, we constructed a chimeric ribozyme with improved catalytic ability and S-domain/C-domain assembly and developed a pair of bimolecular RNase P ribozymes the assembly of which was considerably orthogonal to each other.

Critical domain interactions for type A RNase P RNA catalysis with and without the specificity domain

The natural trans-acting ribozyme RNase P RNA (RPR) is composed of two domains in which the catalytic (C-) domain mediates cleavage of various substrates. The C-domain alone, after removal of the second specificity (S-) domain, catalyzes this reaction as well, albeit with reduced efficiency. Here we provide experimental evidence indicating that efficient cleavage mediated by the Escherichia coli C-domain (Eco CP RPR) with and without the C5 protein likely depends on an interaction referred to as the "P6-mimic". Moreover, the P18 helix connects the C- and S-domains between its loop and the P8 helix in the S-domain (the P8/ P18-interaction). In contrast to the "P6-mimic", the presence of P18 does not contribute to the catalytic performance by the C-domain lacking the S-domain in cleavage of an all ribo model hairpin loop substrate while deletion or disruption of the P8/ P18-interaction in full-size RPR lowers the catalytic efficiency in cleavage of the same model hairpin loop substrate in keeping with previously reported data using precursor tRNAs. Consistent with that P18 is not required for cleavage mediated by the C-domain we show that the archaeal Pyrococcus furiosus RPR C-domain, which lacks the P18 helix, is catalytically active in trans without the S-domain and any protein. Our data also suggest that the S-domain has a larger impact on catalysis for E. coli RPR compared to P. furiosus RPR. Finally, we provide data indicating that the absence of the S-domain and P18, or the P8/ P18-interaction in full-length RPR influences the charge distribution near the cleavage site in the RPR-substrate complex to a small but reproducible extent.

Bacterial type B RNase P: functional characterization of the L5.1-L15.1 tertiary contact and antisense inhibition

Ribonuclease P is the ubiquitous endonuclease that generates the mature 5'-ends of precursor tRNAs. In bacteria, the enzyme is composed of a catalytic RNA (∼400 nucleotides) and a small essential protein subunit (∼13 kDa). Most bacterial RNase P RNAs (P RNAs) belong to the architectural type A; type B RNase P RNA is confined to the low-G+C Gram-positive bacteria. Here we demonstrate that the L5.1-L15.1 intradomain contact in the catalytic domain of the prototypic type B RNase P RNA of Bacillus subtilis is crucial for adopting a compact functional conformation: Disruption of the L5.1-L15.1 contact by antisense oligonucleotides or mutation reduced P RNA-alone and holoenzyme activity by one to two orders of magnitude in vitro, largely retarded gel mobility of the RNA and further affected the structure of regions P7/P8/P10.1, P15 and L15.2, and abolished the ability of B. subtilis P RNA to complement a P RNA-deficient Escherichia coli strain. We also provide mutational evidence that an L9-P1 tertiary contact, as found in some Mycoplasma type B RNAs, is not formed in canonical type B RNAs as represented by B. subtilis P RNA. We finally explored the P5.1 and P15 stem-loop structures as targets for LNA-modified antisense oligonucleotides. Oligonucleotides targeting P15, but not those directed against P5.1, were found to efficiently anneal to P RNA and to inhibit activity (IC₅₀ of ∼2 nM) when incubated with preassembled B. subtilis RNase P holoenzymes.

Modeling the Thermoproteaceae RNase P RNA

The RNA component of the RNase P complex is found throughout most branches of the tree of life and is principally responsible for removing the 5' leader sequence from pre-tRNA transcripts during tRNA maturation. RNase P RNA has a number of universal core features, however variations in sequence and structure found in homologs across the tree of life require multiple Rfam covariance search models to detect accurately. We describe a new Rfam search model to enable efficient detection of the diminutive archaeal Type T RNase P RNAs, which are missed by existing Rfam models. Using the new model, we establish effective score detection thresholds, and detect four new RNase P RNA genes in recently completed genomes from the crenarchaeal family Thermoproteaceae.

Interactions of a Pop5/Rpp1 heterodimer with the catalytic domain of RNase MRP

Ribonuclease (RNase) MRP is a multicomponent ribonucleoprotein complex closely related to RNase P. RNase MRP and eukaryotic RNase P share most of their protein components, as well as multiple features of their catalytic RNA moieties, but have distinct substrate specificities. While RNase P is practically universally found in all three domains of life, RNase MRP is essential in eukaryotes. The structural organizations of eukaryotic RNase P and RNase MRP are poorly understood. Here, we show that Pop5 and Rpp1, protein components found in both RNase P and RNase MRP, form a heterodimer that binds directly to the conserved area of the putative catalytic domain of RNase MRP RNA. The Pop5/Rpp1 binding site corresponds to the protein binding site in bacterial RNase P RNA. Structural and evolutionary roles of the Pop5/Rpp1 heterodimer in RNases P and MRP are discussed.