TYMV RNA as a substrate of tRNA maturation endonuclease

Improving the trans-ancestry portability of polygenic risk scores by prioritizing variants in predicted cell-type-specific regulatory elements

Poor trans-ancestry portability of polygenic risk scores is a consequence of Eurocentric genetic studies and limited knowledge of shared causal variants. Leveraging regulatory annotations may improve portability by prioritizing functional over tagging variants. We constructed a resource of 707 cell-type-specific IMPACT regulatory annotations by aggregating 5,345 epigenetic datasets to predict binding patterns of 142 transcription factors across 245 cell types. We then partitioned the common SNP heritability of 111 genome-wide association study summary statistics of European (average n ≈ 189,000) and East Asian (average n ≈ 157,000) origin. IMPACT annotations captured consistent SNP heritability between populations, suggesting prioritization of shared functional variants. Variant prioritization using IMPACT resulted in increased trans-ancestry portability of polygenic risk scores from Europeans to East Asians across all 21 phenotypes analyzed (49.9% mean relative increase in R2). Our study identifies a crucial role for functional annotations such as IMPACT to improve the trans-ancestry portability of genetic data.

Functionally informed fine-mapping and polygenic localization of complex trait heritability

Fine-mapping aims to identify causal variants impacting complex traits. We propose PolyFun, a computationally scalable framework to improve fine-mapping accuracy by leveraging functional annotations across the entire genome-not just genome-wide-significant loci-to specify prior probabilities for fine-mapping methods such as SuSiE or FINEMAP. In simulations, PolyFun + SuSiE and PolyFun + FINEMAP were well calibrated and identified >20% more variants with a posterior causal probability >0.95 than identified in their nonfunctionally informed counterparts. In analyses of 49 UK Biobank traits (average n = 318,000), PolyFun + SuSiE identified 3,025 fine-mapped variant-trait pairs with posterior causal probability >0.95, a >32% improvement versus SuSiE. We used posterior mean per-SNP heritabilities from PolyFun + SuSiE to perform polygenic localization, constructing minimal sets of common SNPs causally explaining 50% of common SNP heritability; these sets ranged in size from 28 (hair color) to 3,400 (height) to 2 million (number of children). In conclusion, PolyFun prioritizes variants for functional follow-up and provides insights into complex trait architectures.

The archaeology of coding RNA

Different theories concerning the origin of RNA (and, in particular, mRNA) point to the concatenation and expansion of proto-tRNA-like structures. Different biochemical and biophysical tools have been used to search for ancient-like RNA elements with a specific structure in genomic viral RNAs, including that of the hepatitis C virus, as well as in cellular mRNA populations, in particular those of human hepatocytes. We define this method as "archaeological," and it has been designed to discover evolutionary patterns through a nonphylogenetic and nonrepresentational strategy. tRNA-like elements were found in structurally or functionally relevant positions both in viral RNA and in one of the liver mRNAs examined, the antagonist interferon-alpha subtype 5 (IFNA5) mRNA. Additionally, tRNA-like elements are highly represented within the hepatic mRNA population, which suggests that they could have participated in the formation of coding RNAs in the distant past. Expanding on this finding, we have observed a recurring dsRNA-like motif next to the tRNA-like elements in both viral RNAs and IFNA5 mRNA. This suggested that the concatenation of these RNA motifs was an activity present in the RNA pools that might have been relevant in the RNA world. The extensive alteration of sequences that likely triggered the transition from the predecessors of coding RNAs to the first fully functional mRNAs (which was not the case in the stepwise construction of noncoding rRNAs) hinders the phylogeny-based identification of RNA elements (both sequences and structures) that might have been active before the advent of protein synthesis. Therefore, our RNA archaeological method is presented as a way to better understand the structural/functional versatility of a variety of RNA elements, which might represent "the losers" in the process of RNA evolution as they had to adapt to the selective pressures favoring the coding capacity of the progressively longer mRNAs.

Viral tRNA Mimicry from a Biocommunicative Perspective

RNA viruses have very small genomes which limits the functions they can encode. One of the strategies employed by these viruses is to mimic key factors of the host cell so they can take advantage of the interactions and activities these factors typically participate in. The viral RNA genome itself was first observed to mimic cellular tRNA over 40 years ago. Since then researchers have confirmed that distinct families of RNA viruses are accessible to a battery of cellular factors involved in tRNA-related activities. Recently, potential tRNA-like structures have been detected within the sequences of a 100 mRNAs taken from human cells, one of these being the host defense interferon-alpha mRNA; these are then additional to the examples found in bacterial and yeast mRNAs. The mimetic relationship between tRNA, cellular mRNA, and viral RNA is the central focus of two considerations described below. These are subsequently used as a preface for a final hypothesis drawing on concepts relating to mimicry from the social sciences and humanities, such as power relations and creativity. Firstly, the presence of tRNA-like structures in mRNAs indicates that the viral tRNA-like signal could be mimicking tRNA-like elements that are contextualized by the specific carrier mRNAs, rather than, or in addition to, the tRNA itself, which would significantly increase the number of potential semiotic relations mediated by the viral signals. Secondly, and in particular, mimicking a host defense mRNA could be considered a potential new viral strategy for survival. Finally, we propose that mRNA's mimicry of tRNA could be indicative of an ancestral intracellular conflict in which species of mRNAs invaded the cell, but from within. As the meaning of the mimetic signal depends on the context, in this case, the conflict that arises when the viral signal enters the cell can change the meaning of the mRNAs' internal tRNA-like signals, from their current significance to that they had in the distant past.

Entrapping Ribosomes for Viral Translation

Turnip yellow mosaic virus (TYMV) has a genomic plus-strand RNA with a 5' cap followed by overlapping and different reading frames for the movement protein and polyprotein, while the distal coat protein cistron is translated from a subgenomic RNA. The 3'-untranslated region harbors a tRNA-like structure (TLS) to which a valine moiety can be added and it is indispensable for virus viability. Here, we report about a surprising interaction between TYMV-RNA-programmed ribosomes and 3'-valylated TLS that yields polyprotein with the valine N terminally incorporated by a translation mechanism resistant to regular initiation inhibitors. Disruption of the TLS exclusively abolishes polyprotein synthesis, which can be restored by adding excess TLS in trans. Our observations imply a novel eukaryotic mechanism for internal initiation of mRNA translation.

The TYMV tRNA-like structure

The genomic RNA from turnip yellow mosaic virus presents a 3'-end functionally and structurally related to tRNAs. This report summarizes our knowledge about the peculiar structure of the tRNA-like domain and its interaction with tRNA specific proteins, like RNAse P, tRNA nucleotidyl-transferase, aminoacyl-tRNA synthetases, and elongation factors. It discusses also the biological role of this structure in the viral life cycle. A brief survey of our knowledge of other tRNA mimicries in biological systems, as well as their relevance for understanding canonical tRNA, will also be presented.

Novel reactions of RNAase P with a tRNA-like structure in turnip yellow mosaic virus RNA

A quasi-continuous double hellix, containing a pseudoknot and ending in a single-stranded region which contains CCA, can be formed at the 3' terminus of the genomic RNAs of certain plant viruses. M1 RNA (the catalytic subunit) alone and the RNAase P holoenzyme from E. coli cleave the tRNA-like structure of TYMV RNA in vitro at the 5' side of the quasi-helical structure to generate 5' phosphate and 3' hydroxyl groups in the cleavage products. The intact pseudoknot structure in the substrate is not required for the reaction catalyzed by M1 RNA alone, but its presence markedly improves the efficiency of the reaction. In addition to its essential role in the biosynthesis of tRNA, RNAase P may have another function in vivo, namely, in the physiology of viral infections.

Length requirements for tRNA-specific enzymes and cleavage specificity at the 3' end of turnip yellow mosaic virus RNA

This paper describes the minimum length of the turnip yellow mosaic virus (TYMV) RNA necessary to fulfill the tRNA-like properties of the viral RNA: 50 to 75 nucleotides and 86 nucleotides from the 3' end of TYMV RNA are sufficient for adenylation and valylation respectively by the Escherichia coli system. The size of the tRNA-like fragments obtained in vitro in the presence of an E. coli, a reticulocyte or a chinese cabbage leaf extract has also been determined. Among the major fragments liberated from the 3' end of TYMV RNA by the three systems are fragments of 117 and 112 nucleotides. In addition, the E. coli extract liberates fragments of 139 and 61 nucleotides, and the reticulocyte lysate fragments of 109, 94, 84, 73 and 46 nucleotides. The cleavage of the viral RNA by several systems in vitro to yield RNA fragments encompassing the tRNA-like sequence suggests that such fragments might also be liberated in vivo.

Large-scale purification of the 3'-OH-terminal tRNA-like sequence (n = 159) of turnip-yellow-mosaic-virus RNA

In order to undertake structural and functional studies on the 3'-terminal part of turnip yellow mosaic virus RNA, a structure which can be specifically aminoacylated by valyl-tRNA synthetase, we have developed large-scale methods for purifying the tRNA-like sequence. Several experimental approaches were tested. One procedure was retained enabling us to purify large quantities of the homogeneous tRNA-like fragment. Starting from 1.5 g turnip yellow mosaic virus, one obtains 400 mg RNA, which is partially digested by T1 ribonuclease and which yields 1-2 mg pure tRNA-like fragment after three chromatographic steps: two filtrations on Ultrogel ACA 54 and one reverse-phase chromatography (RPC 5) in the presence of urea. A method has been worked out allowing preparation of 10 mg of the fragment per month. The purified RNA material appeared homogeneous upon polyacrylamide gel electrophoresis under denaturing conditions. The isolated tRNA-like structure can be valylated to an extent of 100% in the presence of purified yeast valyl-tRNA synthetase with kinetic parameters resembling those of the tRNAVal aminoacylation.

In vitro methylation of tobacco mosaic virus RNA with ribothymidine-forming tRNA methyltransferase. Characterization and specificity of the reaction

A novel method has been developed for the detection and study of tRNA-like moieties in viral RNAs. Tobacco mosaic virus RNA is an acceptable substrate for crude Escherichia coli ribothymidine-forming tRNA methyltransferase. Under optimum reaction conditions at least 85% of the methylation product is ribothymidine (rT). The reaction is essentially quantitative, 1 mol of rT being formed per mol of tobacco mosaic virus RNA. The optimum reaction conditions include the presence of 6.6 micrometers S-adenosyl-L-[Me-3H]methionine, 25 micrometers spermine, 25 mM ammonium acetate, and 50 mM HEPES, pH 8.0. Sequence analysis of (Me-3H)-labeled tobacco mosaic virus RNA shows that all of the methylation occurs at a single site and strongly suggests that this site is the 32nd residue from the 3'-end of tobacco mosaic virus RNA. This site closely resembles the normal position of rT in transfer RNA.

Purification and properties of tRNA nucleotidyl transferase from Musca domestica

The enzyme tRNA nucleotidyl transferase (EC 2.7.7.25) has been highly purified from whole adult houseflies. A molecular weight of 30 000 has been determined. The enzyme requires Mg2+ and tRNA deprived of the 3' terminal sequence CCA for activity in the incorporation of AMP and CMP onto the tRNA. UMP can be incorporated instead of CMP but the latter has a higher affinity than UMP as shown by competition experiments. A complex between the enzyme and tRNA has been shown by sucrose gradient centrifugation, nitrocellulose binding and protection by tRNA against enzyme denaturation at 50 degrees C. Comparative studies with tRNA nucleotidyl transferase purified from larvae, pupae and adult insects indicate that tRNA nucleotidyl transferase from these three developmental stages have the same molecular weight, sedimentation coefficient and optimum pH while the larval enzyme differs from the pupae and adult enzymes in the elution behaviour from a DEAE-cellulose column.

Nucleotide sequence (n=159) of the amino-acid-accepting 3'-OH extremity of turnip-yellow-mosaic-virus RNA and the last portion of its coat-protein cistron

The experiments described in this paper and the following one establish the sequence of the 3'-OH terminal 159 nucleotides of turnip yellow mosaic virus RNA. Uniformly 32P-labeled turnip yellow mosaic virus RNA was partially digested with T1 ribonuclease and the fragments were fractionated by polyacrylamide gel electrophoresis. Fragments originating from the 3'-OH end of the RNA molecule were identified by testing for the 3'-terminal oligonucleotide, C-COH, after total U2 ribonuclease hydrolysis. Once identified, the 3'-OH terminal fragments were sequenced by the methods of Sanger et al. The first 51 nucleotides of the longest of the sequenced fragments (158 nucleotides) extends into the 3'-terminal part of the coat protein cistron. The coat protein cistron is followed by a stretch of 108 untranslated nucleotides whose function, though still unknown, is probably linked to the tRNA-like properties which have been attributed to the 3'-OH extremity of this viral RNA. Two possible secondary structures are proposed for the sequence and the implications of the findings with regard to the tRNA-like properties of the extremity are discussed.

Studies on the sequence of the 3'-terminal region of turnip-yellow-mosaic-virus RNA

A fragment representing the 3'-terminal 'tRNA-like' region of turnip yellow mosaic (TYM) virus RNA has been purified following incubation of intact TYM virus RNA with Escherichia coli 'RNase P'. This fragment, which is 112+3-nucleotides long has been completely digested with T1 RNase and pancreatic RNase and all the oligonucleotides present in such digests have been sequenced using 32P-end labelling techniques in vitro. The TYM virus RNA fragment is free of modified nucleosides and does not contain a G-U-U-C-R sequence. Using nuclease P1 from Penicillium citrinum, the sequence of 26 nucleotides from the 5' end and 16 nucleotides from the 3' end of this fragment has been deduced. The nucleotide sequence at the 5' end of the TYM virus RNA fragment indicates that this fragment includes the end of the TYM virus coat protein gene.

Structural features of encephalomyocarditis virus RNA from analysis of reverse transcription products

The presence in encephalomyocarditis (EMC) virus RNA of homonucleotide tracts 10 nucleotides or more in length has been investigated by testing the ability of homo-oligodeoxynucleotides to prime DNA synthesis in the reverse transcriptase from avian myeloblastosis virus. Neither (dC)10 nor (dA)10 promoted incorporation of [3H]deoxynucleotides into acid-insoluble material but (dG)10 and (dT)12-18 were effective primers and produced DNA products approximately 2000 nucleotides in length. We conclude that there are single-stranded oligo(rC) and oligo(rA) tracts in native EMC virus RNA at 37 degrees C. Kinetic analysis indicated that oligo(dT) priming is similar to priming on ovalbumin mRNA and that it gives rise to only one DNA product per template molecule. Oligo(dG) priming appears to be complicated by self-aggregation of the primer. Oligo(dT)-primed and oligo(dG)-primed DNA have both been separated on alkaline-sucrose gradients into two peaks of which only the 'heavier' will hybridise to EMC virus RNA. Competitive hybridisation experiments indicate that the 'heavy' oligo(dT)-primed and oligo(dG)-primed DNA fractions hybridise to overlapping sequences of EMC virus RNA and place the priming regions of EMC virus RNA approximately 500 nucleotides apart during reverse transcription.

Patterns of E. coli leucine tRNA isoacceptors following bacteriophage MS2 infection

The RNA extracted from MS2 phage particles can accept radioactive leucine and serine in the presence of tRNA activating enzymes. Leucine acceptance is due to the presence of E. coli leucine tRNA that binds very tightly to the virus particle. RPC-5 column chromatography shows that the pattern of virus associated leucyl-tRNA isoacceptors is different from that of normal E. coli leucyl-tRNA. It is also different from the pattern of host leucyl-tRNA isoacceptors found in E. coli lysate following MS2 phage infection. The RPC-5 pattern of the latter tRNA shows several new peaks of leucine tRNA isoacceptors. The possibility that these tRNAs are some modified forms of normal leucine tRNA isoacceptors is suggested.

tRNA nucleotidyltransferase-catalyzed incorporation of CMP and AMP into RNA-bacteriophage genome fragments

Fragments of bacteriophage RNAs R17, MS2 and Qbeta obtained by incubation with commercial snake venom phosphodiesterase become substrates of the Escherichia coli tRNA nucleotidyltransferase. The transferase adds back CMP and AMP in conditions in which it remains highly specific of CCA-deprived tRNAs. The results suggest that the fragment from the 3' end of the viral genome and/or possibly one or more internal fragment(s) are recognized by the transferase. These observations might indicate that bacteriophage RNAs contain certain features probably present in all tRNAs and which are recognized by the transferase.

Biosynthesis of transfer RNA: in vitro conversion of transfer RNA precursors from Bombyx mori to 4S RNA by Escherichia coli enzymes

Evidence for the presence of rapidly labeled short-lived RNA species, presumed to be precursors to transfer RNA (tRNA), in the silk glands of Bombyx mori is presented. These precursors, which migrate between 4S and 5S markers on acrylamide gels, can be converted in vitro into molecules indistinguishable in size from tRNA. The conversion is also catalyzed by a crude enzyme fraction isolated from ribosomes of E. coli. Since a similar enzyme preparation cleaves the suppressor tRNA(Tyr) sequence of E. coli from its precursor molecule, it would appear that the precursor to E. coli tRNA(Tyr) and the precursors to silk-gland tRNAs share common structural features.