Mature Rotavirus Particles Contain Equivalent Amounts of 7meGpppG-Capped and Noncapped Viral Positive-Sense RNAs.

Viruses have evolved different strategies to overcome their recognition by the host innate immune system. The addition of caps at their 5' RNA ends is an efficient mechanism not only to ensure escape from detection by the innate immune system but also to ensure the efficient synthesis of viral proteins. Rotavirus mRNAs contain a type 1 cap structure at their 5' end that is added by the viral capping enzyme VP3, which is a multifunctional protein with all the enzymatic activities necessary to add the cap and also functions as an antagonist of the 2'-5'-oligoadenylate synthetase (OAS)/RNase L pathway. Here, the relative abundances of capped and noncapped viral RNAs during the replication cycle of rotavirus were determined. We found that both classes of rotaviral plus-sense RNAs (+RNAs) were encapsidated and that they were present in a 1:1 ratio in the mature infectious particles. The capping of viral +RNAs was dynamic, since different ratios of capped and noncapped RNAs were detected at different times postinfection. Similarly, when the relative amounts of capped and uncapped viral +RNAs produced in an in vitro transcription system were determined, we found that the proportions were very similar to those in the mature viral particles and in infected cells, suggesting that the capping efficiency of VP3, both in vivo and in vitro, might be close to 50%. Unexpectedly, when the effect of simultaneously knocking down the expression of VP3 and RNase L on the cap status of viral +RNAs was evaluated, we found that, even though at late times postinfection there was an increased proportion of capped viral RNAs in infected cells, the viral particles isolated from this condition contained equal ratios of capped and noncapped viral RNA, suggesting that there might be selective packaging of capped and noncapped RNAs. IMPORTANCE Rotaviruses have a genome composed of 11 segments of double-stranded RNA. Whether all 5' ends of the positive-sense genomic RNAs contained in the mature viral particles are modified by a cap structure is unknown. In this work, we characterized the relative proportions of capped and noncapped viral RNAs in rotavirus-infected cells and in viral particles by using a direct quantitative assay. We found that, independent of the relative proportions of capped/noncapped RNAs present in rotavirus-infected cells, there were similar proportions of these two kinds of 5'-modified positive-sense RNAs in the viral particles.

Chemo-Enzymatic Modification of the 5' Cap Maintains Translation and Increases Immunogenic Properties of mRNA.

Eukaryotic mRNAs are emerging modalities for protein replacement therapy and vaccination. Their 5' cap is important for mRNA translation and immune response and can be naturally methylated at different positions by S-adenosyl-l-methionine (AdoMet)-dependent methyltransferases (MTases). We report on the cosubstrate scope of the MTase CAPAM responsible for methylation at the N6 -position of adenosine start nucleotides using synthetic AdoMet analogs. The chemo-enzymatic propargylation enabled production of site-specifically modified reporter-mRNAs. These cap-propargylated mRNAs were efficiently translated and showed ≈3-fold increased immune response in human cells. The same effects were observed when the receptor binding domain (RBD) of SARS-CoV-2-a currently tested epitope for mRNA vaccination-was used. Site-specific chemo-enzymatic modification of eukaryotic mRNA may thus be a suitable strategy to modulate translation and immune response of mRNAs for future therapeutic applications.

CapZyme-Seq: A 5'-RNA-Seq Method for Differential Detection and Quantitation of NAD-Capped and Uncapped 5'-Triphosphate RNA.

Nucleoside-containing metabolites such as the oxidized and reduced forms of nicotinamide adenine dinucleotide (NAD+ and NADH), 3'-desphospho-coenzyme A (dpCoA), and flavin adenine dinucleotide (FAD) can be incorporated as RNA 5' end caps by serving as non-canonical initiating nucleotides (NCINs) for transcription initiation by RNA polymerase. We recently reported ″CapZyme-seq,″ a 5'-RNA-seq method that enables the differential detection and quantitation of relative yields of NCIN-capped RNA and uncapped 5'-triphosphate RNA. Here we provide the protocol for constructing cDNA libraries for CapZyme-seq. For complete information on the generation and use of this protocol, please refer to Vvedenskaya et al. (2018a).

DXO/Rai1 enzymes remove 5'-end FAD and dephospho-CoA caps on RNAs.

In eukaryotes, the DXO/Rai1 enzymes can eliminate most of the incomplete and non-canonical NAD caps through their decapping, deNADding and pyrophosphohydrolase activities. Here, we report that these enzymes can also remove FAD and dephospho-CoA (dpCoA) non-canonical caps from RNA, and we have named these activities deFADding and deCoAping. The crystal structures of mammalian DXO with 3'-FADP or CoA and fission yeast Rai1 with 3'-FADP provide elegant insight to these activities. FAD and CoA are accommodated in the DXO/Rai1 active site by adopting folded conformations. The flavin of FAD and the pantetheine group of CoA contact the same region at the bottom of the active site tunnel, which undergoes conformational changes to accommodate the different cap moieties. We have developed FAD-capQ to detect and quantify FAD-capped RNAs and determined that FAD caps are present on short RNAs (with less than ∼200 nucleotides) in human cells and that these RNAs are stabilized in the absence of DXO.

CAP-MAP: cap analysis protocol with minimal analyte processing, a rapid and sensitive approach to analysing mRNA cap structures.

Eukaryotic messenger RNA (mRNA) is modified by the addition of an inverted guanosine cap to the 5' triphosphate. The cap guanosine and initial transcribed nucleotides are further methylated by a series of cap methyltransferases to generate the mature cap structures which protect RNA from degradation and recruit proteins involved in RNA processing and translation. Research demonstrating that the cap methyltransferases are regulated has generated interest in determining the methylation status of the mRNA cap structures present in cells. Here, we present CAP-MAP: cap analysis protocol with minimal analyte processing, a rapid and sensitive method for detecting cap structures present in mRNA isolated from tissues or cultured cells.

Analysis of RNA 5' ends: Phosphate enumeration and cap characterization.

The function and fate of cellular RNAs are often governed by the phosphorylation state at the 5' end or the identity of whatever cap may be present there. Here we describe methods for examining these important 5'-terminal features on any cellular or synthetic RNA of interest that can be detected by Northern blotting. One such method, PABLO, is a splinted ligation assay that makes it possible to accurately quantify the percentage of 5' ends that are monophosphorylated. Another, PACO, is a capping assay that reveals the percentage of 5' ends that are diphosphorylated. A third, boronate gel electrophoresis in conjunction with deoxyribozyme-mediated cleavage, enables different types of caps (e.g., m7Gppp caps versus NAD caps) to be distinguished from one another and the percentage of each to be determined. After completing all three tests, the percentage of 5' ends that are triphosphorylated can be deduced by process of elimination. Together, this battery of assays allows the 5' terminus of an RNA to be profiled in unprecedented detail.

Sequencing the cap-snatching repertoire of H1N1 influenza provides insight into the mechanism of viral transcription initiation.

The influenza polymerase cleaves host RNAs ∼10-13 nucleotides downstream of their 5' ends and uses this capped fragment to prime viral mRNA synthesis. To better understand this process of cap snatching, we used high-throughput sequencing to determine the 5' ends of A/WSN/33 (H1N1) influenza mRNAs. The sequences provided clear evidence for nascent-chain realignment during transcription initiation and revealed a strong influence of the viral template on the frequency of realignment. After accounting for the extra nucleotides inserted through realignment, analysis of the capped fragments indicated that the different viral mRNAs were each prepended with a common set of sequences and that the polymerase often cleaved host RNAs after a purine and often primed transcription on a single base pair to either the terminal or penultimate residue of the viral template. We also developed a bioinformatic approach to identify the targeted host transcripts despite limited information content within snatched fragments and found that small nuclear RNAs and small nucleolar RNAs contributed the most abundant capped leaders. These results provide insight into the mechanism of viral transcription initiation and reveal the diversity of the cap-snatched repertoire, showing that noncoding transcripts as well as mRNAs are used to make influenza mRNAs.

CAGE (cap analysis of gene expression): a protocol for the detection of promoter and transcriptional networks.

We provide here a protocol for the preparation of cap-analysis gene expression (CAGE) libraries, which allows for measuring the expression of eukaryotic capped RNAs and simultaneously map the promoter regions. The presented protocol simplifies the previously published ones and moreover produces tags that are 27 nucleotides long, which facilitates mapping to the genome. The protocol takes less than 5 days to complete and presents a notable improvement compared to previously published versions.

Requirement of TFIIH kinase subunit Mat1 for RNA Pol II C-terminal domain Ser5 phosphorylation, transcription and mRNA turnover.

The relevance of serine 5 phosphorylation of RNA polymerase II carboxy-terminal domain during initiation has been difficult to determine in mammalian cells as no general in vivo Ser5 kinase has been identified. Here, we demonstrate that deletion of the TFIIH kinase subunit Mat1 in mouse fibroblasts leads to dramatically reduced Pol II Ser5 phosphorylation. This is associated with defective capping and reduced Ser2 phosphorylation, decreased Pol II progression into elongation and severely attenuated transcription detected through analysis of nascent mRNAs, establishing a general requirement for mammalian Mat1 in transcription. Surprisingly, the general defect in Pol II transcription in Mat1(-/-) fibroblasts is not reflected in the majority of steady-state mRNAs. This indicates widespread stabilization of mRNAs and points to the existence of a regulatory mechanism to stabilize mRNAs following transcriptional attenuation, thus revealing a potential caveat in similar studies limited to analysis of steady-state mRNAs.

A quantitative assay for measuring mRNA decapping by splinted ligation reverse transcription polymerase chain reaction: qSL-RT-PCR.

The degradation of messenger RNA is a critical node of gene regulation. A major pathway of mRNA decay is initiated by shortening of the poly(A) tail, followed by removal of the 5' cap structure (decapping) and subsequent degradation. Decapping is an important determinate in the destruction of many transcripts. Detailed kinetic analysis of in vivo decapping rates is necessary to understand how this step is regulated. Importantly, the product of decapping is recalcitrant for investigation, in part due to its transient nature. As such, little in vivo kinetic information is available. Here we report the development of an assay that measures decapping of mRNAs by combining splinted ligation and quantitative RT-PCR (qSL-RT-PCR). We apply this method to determine the decapping rate constant for a natural mRNA in vivo for the first time. The qSL-RT-PCR assay may be adapted for use on any mRNA, providing a new tool to study regulation of mRNA decay.

Analysis of mRNA decapping.

The modulation of mRNA decay is a critical determinant in the regulation of gene expression. mRNAs in eukaryotes are primarily degraded by two major exonucleolytic pathways: the 5' to 3'-and the 3' to 5'-pathways, both of which are initiated by removal of the polyadenylated (poly(A)) tail. Hydrolysis of the 5'-cap structure, termed decapping, is a key step in the demise of mRNA. Two major decapping enzymes with distinct activities and substrate requirements have been identified. Dcp2 hydrolyzes the cap structure on an intact mRNA in the 5' to 3'-decay pathway; Dcp2 scavenges the residual cap oligonucleotide resulting from the 3' to 5'-decay pathway, as well as hydrolyzes the decapping product generated by Dcp2. In this chapter, we describe the methods for monitoring Dcp2 and DcpS decapping activities of bacterially expressed and endogenous human decapping enzymes.

A kinetic assay to monitor RNA decapping under single- turnover conditions.

The stability of all RNA polymerase II transcripts depends on the 5'-terminal cap structure. Removal of the cap is a prerequisite for 5' to 3'-decay and is catalyzed by distinct cellular and viral decapping activities. Over the past decade, several decapping enzymes have been characterized through functional and structural studies. An emerging theme is that function is regulated by protein interactions; however, in vitro assays to dissect the effects on enzyme activity are unavailable. Here we present a kinetic assay to monitor decapping by the heterodimeric yeast Dcp1/Dcp2 complex. Kinetic constants related to RNA binding and the rate of the catalytic step can be determined with recombinant enzyme and cap-radiolabeled RNA substrate, allowing substrate specificity and the role of activating factors to be firmly established.

An uncapped RNA suggests a model for Caenorhabditis elegans polycistronic pre-mRNA processing.

Polycistronic pre-mRNAs from Caenohabditis elegans operons are processed by internal cleavage and polyadenylation to create 3' ends of mature mRNAs. This is accompanied by trans-splicing with SL2 approximately 100 nucleotides downstream of the 3' end formation sites to create the 5' ends of downstream mRNAs. SL2 trans-splicing depends on a U-rich element (Ur), located approximately 70 nucleotides upstream of the trans-splice site in the intercistronic region (ICR), as well as a functional 3' end formation signal. Here we report the existence of a novel gene-length RNA, the Ur-RNA, starting just upstream of the Ur element. The expression of Ur-RNA is dependent on 3' end formation as well as on the presence of the Ur element, but does not require a trans-splice site. The Ur-RNA is not capped, and alteration of the location of the Ur element in either the 5' or 3' direction alters the location of the 5' end of the Ur-RNA. We propose that a 5' to 3' exonuclease degrades the precursor RNA following cleavage at the poly(A) site, stopping when it reaches the Ur element, presumably attributable to a bound protein. Part of the function of this protein can be performed by the MS2 coat protein. Recruitment of coat protein to the ICR in the absence of the Ur element results in accumulation of an RNA equivalent to Ur-RNA, and restores trans-splicing. Only SL1, however, is used. Therefore, coat protein is sufficient for blocking the exonuclease and thereby allowing formation of a substrate for trans-splicing, but it lacks the ability to recruit the SL2 snRNP. Our results also demonstrate that MS2 coat protein can be used as an in vivo block to an exonuclease, which should have utility in mRNA stability studies.

Analysis of yeast trimethylguanosine-capped RNAs by midwestern blotting.

Immuno-detection by 'Midwestern' blotting provides a simple way to identify trimethylguanosine (TMG) capped RNAs. With this technique, over 20 bands are observed when total cellular RNA from Saccharomyces cerevisiae is transferred to a nylon membrane and probed with anti-TMG antibodies. Most, if not all, species known to contain a TMG cap are detected by this method. Only TMG-capped RNAs are detected on Midwestern blots unlike anti-TMG immunoprecipitates. Midwestern blotting is a useful alternative to immunoprecipitation and Northern analysis and may prove to be a better method for determining the relative abundance of capped RNAs. The blots can be reprobed multiple times with labeled antisense oligonucleotides to determine the identity of any TMG-capped species for which the primary sequence or a clone is available. This dual detection capability provides a powerful tool for the analysis of TMG-capped snRNAs and snoRNAs.

Functional mRNA can be generated by RNA polymerase III.

Eukaryotic cellular mRNA is believed to be synthesized exclusively by RNA polymerase II (pol II), whereas pol I produces long rRNAs and pol III produces 5S rRNA, tRNA, and other small RNAs. To determine whether this functional differentiation is obligatory, we examined the translational potential of an artificial pol III transcript. The coding region of the human immunodeficiency virus type 1 tat gene was placed under the control of a strong pol III promoter from the adenovirus type 2 VA RNAI gene. The resultant chimera, pVA-Tat, was transcribed accurately in vivo and in vitro and gave rise to Tat protein, which transactivated a human immunodeficiency virus-driven chloramphenicol acetyltransferase reporter construct in transfected HeLa cells. pol III-specific mutations down-regulated VA-Tat RNA production in vivo and in vitro and dramatically reduced chloramphenicol acetyltransferase transactivation. As expected for a pol III transcript, VA-Tat RNA was not detectably capped at its 5' end or polyadenylated at its 3' end, but, like mRNA, it was associated with polysomes in a salt-stable manner. Mutational analysis of a short open reading frame upstream of the Tat-coding sequence implicates scanning in the initiation of VA-Tat RNA translation despite the absence of a cap. In comparison with tat mRNA generated by pol II, VA-Tat RNA was present on smaller polysomes and was apparently translated less efficiently, which is consistent with a relatively low initiation rate. Evidently, human cells are capable of utilizing pol III transcripts as functional mRNAs, and neither a cap nor a poly(A) tail is essential for translation, although they may be stimulatory. These findings raise the possibility that some cellular mRNAs are made by pol I or pol III.

Characterization of three new snRNAs from Saccharomyces cerevisiae: snR34, snR35 and snR36.

Genes for three novel snRNAs of Saccharomyces cerevisiae have been isolated, sequenced and tested for essentiality. The RNAs encoded by these genes are designated snR34, snR35 and snR36 respectively and contain 203, 204 and 182 nucleotides. Each RNA is derived from a single copy gene and all three RNAs are believed to be nucleolar, i.e. snoRNAs, based on extraction properties and association with fibrillarin. SnR34 and snR35 contain a trimethylguanosine cap, but this feature is absent from snR36. The novel RNAs lack elements conserved among several other snoRNAs, including box C, box D and long sequence complementarities with rRNA. Genetic disruption analyses showed each of the RNAs to be dispensable and a haploid strain lacking all three RNAs and a previously characterized fourth snoRNA (snR33) is also viable. No differences in the levels of precursors or mature rRNAs were apparent in the four gene knock-out strain. Possible roles for the new RNAs in ribosome biogenesis are discussed.

Three novel functional variants of human U5 small nuclear RNA.

We have identified and characterized three new variants of U5 small nuclear RNA (snRNA) from HeLa cells, called U5D, U5E, and U5F. Each variant has a 2,2,7-trimethylguanosine cap and is packaged into an Sm-precipitable small nuclear ribonucleoprotein (snRNP) particle. All retain the evolutionarily invariant 9-base loop at the top of stem 1; however, numerous base changes relative to the abundant forms of U5 snRNA are present in other regions of the RNAs, including a loop that is part of the yeast U5 minimal domain required for viability and has been shown to bind a protein in HeLa extracts. U5E and U5F each constitute 7% of the total U5 population in HeLa cells and are slightly longer than the previously characterized human U5 (A, B, and C) species. U5D, which composes 5% of HeLa cell U5 snRNAs, is present in two forms: a full-length species, U5DL, and a shorter species, U5DS, which is truncated by 15 nucleotides at its 3' end and therefore resembles the short form of U5 (snR7S) in Saccharomyces cerevisiae. We have established conditions that allow specific detection of the individual U5 variants by either Northern blotting (RNA blotting) or primer extension; likewise, U5E and U5F can be specifically and completely degraded in splicing extracts by oligonucleotide-directed RNase H cleavage. All variant U5 snRNAs are assembled into functional particles, as indicated by their immunoprecipitability with anti-(U5) RNP antibodies, their incorporation into the U4/U5/U6 tri-snRNP complex, and their presence in affinity-purified spliceosomes. The higher abundance of these U5 variants in 293 cells compared with that in HeLa cells suggests possible roles in alternative splicing.

Polypeptide components of Drosophila small nuclear ribonucleoprotein particles.

In eukaryotes splicing of pre-mRNAs is mediated by the spliceosome, a dynamic complex of small nuclear ribonucleoprotein particles (snRNPs) that associate transiently during spliceosome assembly and the splicing reaction. We have purified snRNPs from nuclear extracts of Drosophila cells by affinity chromatography with an antibody specific for the trimethylguanosine (m3G) cap structure of snRNAs U1-U5. The polypeptide components of Drosophila snRNPs have been characterized and shown to consist of a number of proteins shared by all the snRNPs, and some proteins which appear to be specific to individual snRNP particles. On the basis of their apparent molecular weight and antigenicity many of these common and particle specific Drosophila snRNP proteins are remarkably conserved between Drosophila and human spliceosomes. By probing western blots of the Drosophila snRNP polypeptides with a number of antisera raised against human snRNP proteins, Drosophila polypeptides equivalent to many of the HeLa snRNP-common proteins have been identified, as well as candidates for a number of U1, U2 and U5-specific proteins.