Pseudouridylation (Psi) of U2 snRNA in S. cerevisiae is catalyzed by an RNA-independent mechanism

Functionality and substrate specificity of human box H/ACA guide RNAs

A large number of box H/ACA RNAs have been identified in human cells, and have been predicted to account for nearly all pseudouridylation sites in human rRNAs. However, the function of these mammalian H/ACA RNAs in directing pseudouridylation has been verified experimentally in only two cases. In this study, we used three in vitro reconstitution systems, including yeast and mammalian systems, to test the function of seven H/ACA RNAs guiding16 pseudouridylation sites. Our results verified 12 of these sites; four predictions were incorrect. Further analyses indicated that three components, including the stability of the hairpin structure harboring the pseudouridylation pocket, the stability of guide sequence-target RNA base-pairing interaction, and the distance between the target uridine and the box H or ACA, were critical for the guide function, as changes in these components were sufficient to alter the functionality and specificity of the pseudouridylation pocket. The dynamic functional changes in response to changes in these three important components were further tested in vivo, and the results were completely consistent with the in vitro results. Finally, we compared our results with predictions made by two computer programs, as well as predictions made by human experts using visual inspection. We found that the predictions of one program (snoGPS) agreed with our experimental results with 100% sensitivity (12/12) and 75% specificity (3/4).

RNA sequence and two-dimensional structure features required for efficient substrate modification by the Saccharomyces cerevisiae RNA:{Psi}-synthase Pus7p

The RNA:pseudouridine (Psi) synthase Pus7p of Saccharomyces cerevisiae is a multisite-specific enzyme that is able to modify U(13) in several yeast tRNAs, U(35) in the pre-tRNA(Tyr) (GPsiA), U(35) in U2 small nuclear RNA, and U(50) in 5 S rRNA. Pus7p belongs to the universally conserved TruD-like family of RNA:Psi-synthases found in bacteria, archaea, and eukarya. Although several RNA substrates for yeast Pus7p have been identified, specificity of their recognition and modification has not been studied. However, conservation of a 7-nt-long sequence, including the modified U residue, in all natural Pus7p substrates suggested the importance of these nucleotides for Pus7p recognition and/or catalysis. Using site-directed mutagenesis, we designed a set of RNA variants derived from the yeast tRNA(Asp)(GUC), pre-tRNA(Tyr)(GPsiA), and U2 small nuclear RNA and tested their ability to be modified by Pus7p in vitro. We demonstrated that the highly conserved U(-2) and A(+1) residues (nucleotide numbers refer to target U(0)) are crucial identity elements for efficient modification by Pus7p. Nucleotide substitutions at other surrounding positions (-4, -3, +2, +3) have only a moderate effect. Surprisingly, the identity of the nucleotide immediately 5' to the target U(0) residue (position -1) is not important for efficient modification. Alteration of tRNA three-dimensional structure had no detectable effect on Pus7p activity at position 13. However, our results suggest that the presence of at least one stem-loop structure including or close to the target U nucleotide is required for Pus7p-catalyzed modification.

Proteome-Scale Analysis of Biochemical Activity

During the last 10 years, there has been a large increase in the number of genome sequences available for study, altering the way that the biology of organisms is studied. In particular, scientific attention has increasingly focused on the proteome, and specifically on the role of all the proteins encoded by the genome. We focus here on several aspects of this problem. We describe several technologies in widespread use to clone genes on a genome-wide scale, and to express and purify the proteins encoded by these genes. We also describe a number of methods that have been developed to analyze various biochemical properties of the proteins, with attention to the methodology and the limitations of the approaches, followed by a look at possible developments in the next decade.

RNA-based 5-fluorouracil toxicity requires the pseudouridylation activity of Cbf5p

The chemotherapeutic drug 5-fluorouracil (5FU) disrupts DNA synthesis by inhibiting the enzymatic conversion of dUMP to dTMP. However, mounting evidence indicates that 5FU has important effects on RNA metabolism that contribute significantly to the toxicity of the drug. Strains with mutations in nuclear RNA-processing exosome components, including Rrp6p, exhibit strong 5FU hypersensitivity. Studies also suggest that 5FU-containing RNA can inhibit pseudouridylation, the most abundant post-transcriptional modification of noncoding RNA. We examined the effect of modulating the expression and activity of the essential yeast rRNA pseudouridylase Cbf5p on the 5FU hypersensitivity of an rrp6-delta mutant strain. Depletion of Cbf5p suppressed the 5FU hypersensitivity of an rrp6-delta strain, while high-copy expression enhanced sensitivity to the drug. A mutation in the catalytic site of Cbf5p also suppressed the 5FU hypersensitivity in the rrp6-Delta mutant, suggesting that RNA-based 5FU toxicity requires the pseudouridylation activity of Cbf5p. High-copy expression of box H/ACA snoRNAs also suppressed the 5FU hypersensitivity of an rrp6-delta strain, suggesting that sequestration of Cbf5p to a particular guide RNA reduces Cbf5p-dependent 5FU toxicity. On the basis of these results and previous reports that certain pseudouridylases form stable adducts with 5FU-containing RNA, we suggest that Cbf5p binds tightly to substrates containing 5FU, causing their degradation by the TRAMP/exosome-mediated RNA surveillance pathway.

Different mechanisms for pseudouridine formation in yeast 5S and 5.8S rRNAs

The selection of sites for pseudouridylation in eukaryotic cytoplasmic rRNA occurs by the base pairing of the rRNA with specific guide sequences within the RNA components of box H/ACA small nucleolar ribonucleoproteins (snoRNPs). Forty-four of the 46 pseudouridines (Psis) in the cytoplasmic rRNA of Saccharomyces cerevisiae have been assigned to guide snoRNAs. Here, we examine the mechanism of Psi formation in 5S and 5.8S rRNA in which the unassigned Psis occur. We show that while the formation of the Psi in 5.8S rRNA is associated with snoRNP activity, the pseudouridylation of 5S rRNA is not. The position of the Psi in 5.8S rRNA is guided by snoRNA snR43 by using conserved sequence elements that also function to guide pseudouridylation elsewhere in the large-subunit rRNA; an internal stem-loop that is not part of typical yeast snoRNAs also is conserved in snR43. The multisubstrate synthase Pus7 catalyzes the formation of the Psi in 5S rRNA at a site that conforms to the 7-nucleotide consensus sequence present in other substrates of Pus7. The different mechanisms involved in 5S and 5.8S rRNA pseudouridylation, as well as the multiple specificities of the individual trans factors concerned, suggest possible roles in linking ribosome production to other processes, such as splicing and tRNA synthesis.

SKAR links pre-mRNA splicing to mTOR/S6K1-mediated enhanced translation efficiency of spliced mRNAs

Different protein complexes form on newly spliced mRNA to ensure the accuracy and efficiency of eukaryotic gene expression. For example, the exon junction complex (EJC) plays an important role in mRNA surveillance. The EJC also influences the first, or pioneer round of protein synthesis through a mechanism that is poorly understood. We show that the nutrient-, stress-, and energy-sensing checkpoint kinase, mTOR, contributes to the observed enhanced translation efficiency of spliced over nonspliced mRNAs. We demonstrate that, when activated, S6K1 is recruited to the newly synthesized mRNA by SKAR, which is deposited at the EJC during splicing, and that SKAR and S6K1 increase the translation efficiency of spliced mRNA. Thus, SKAR-mediated recruitment of activated S6K1 to newly processed mRNPs serves as a conduit between mTOR checkpoint signaling and the pioneer round of translation when cells exist in conditions supportive of protein synthesis.

The Saccharomyces cerevisiae Pus2 protein encoded by YGL063w ORF is a mitochondrial tRNA:Psi27/28-synthase

The RNA:pseudouridine (Psi)-synthase family is one of the most complex families of RNA modification enzymes. Ten genes encoding putative RNA:Psi-synthases have been identified in S. cerevisiae. Most of the encoded enzymes have been characterized experimentally. Only the putative RNA:Psi-synthase Pus2p (encoded by the YGL063w ORF) had no identified substrate. Here, we analyzed Psi residues in cytoplasmic and mitochondrial tRNAs extracted from S. cerevisiae strains, carrying disruptions in the PUS1 and/or PUS2 ORFs. Our results demonstrate that Pus2p is a mitochondrial-specific tRNA:Psi-synthase acting at positions 27 and 28 in tRNAs. The importance of the Asp56 residue in the conserved ARTD motif of the Pus2p catalytic site is demonstrated in vivo. Interestingly, in spite of the absence of a characteristic N-terminal targeting signal, our data strongly suggest an efficient and rapid targeting of Pus2p in yeast mitochondria. In contradiction with the commonly held idea that a unique nuclear gene encodes the enzyme required for both cytoplasmic and mitochondrial tRNA modifications, here we show the existence of an enzyme specifically dedicated to mitochondrial tRNA modification (Pus2p), the corresponding modification in cytoplasmic tRNAs being catalyzed by another protein (Pus1p).

Pseudouridine synthases

Pseudouridine synthases are the enzymes responsible for the most abundant posttranscriptional modification of cellular RNAs. These enzymes catalyze the site-specific isomerization of uridine residues that are already part of an RNA chain, and appear to employ both sequence and structural information to achieve site specificity. Crystallographic analyses have demonstrated that all pseudouridine synthases share a common core fold and active site structure and that this core is modified by peripheral domains, accessory proteins, and guide RNAs to give rise to remarkable substrate versatility.

Detection of enzymatic activity of transfer RNA modification enzymes using radiolabeled tRNA substrates

The presence of modified ribonucleotides derived from adenosine, guanosine, cytidine, and uridine is a hallmark of almost all cellular RNA, and especially tRNA. The objective of this chapter is to describe a few simple methods that can be used to identify the presence or absence of a modified nucleotide in tRNA and to reveal the enzymatic activity of particular tRNA-modifying enzymes in vitro and in vivo. The procedures are based on analysis of prelabeled or postlabeled nucleotides (mainly with [(32)P] but also with [(35)S], [(14)C] or [(3)H]) generated after complete digestion with selected nucleases of modified tRNA isolated from cells or incubated in vitro with modifying enzyme(s). Nucleotides of the tRNA digests are separated by two-dimensional (2D) thin-layer chromatography on cellulose plates (TLC), which allows establishment of base composition and identification of the nearest neighbor nucleotide of a given modified nucleotide in the tRNA sequence. This chapter provides useful maps for identification of migration of approximately 70 modified nucleotides on TLC plates by use of two different chromatographic systems. The methods require only a few micrograms of purified tRNA and can be run at low cost in any laboratory.

Identification and Characterization of Modification Enzymes by Biochemical Analysis of the Proteome

The use of proteomic libraries designed to express the complete set of proteins from an organism has resulted in the identification of many RNA modification enzymes whose function was previously unknown. Here we describe a generalized procedure for the biochemical analysis of a yeast proteomic library for identification of nucleic acid-modifying enzymes, by use of the yeast MORF (Moveable Open Reading Frame) library (Gelperin et al., 2005) as the source of protein activity, and the known yeast tRNA methyltransferase Trm4 as a test case. The procedures outlined in this chapter can be applied to any proteomic expression library from any organism, many of which will become increasingly available as the number of sequenced genomes increases and as genomic cloning techniques improve.

Incorporation of 5-fluorouracil into U2 snRNA blocks pseudouridylation and pre-mRNA splicing in vivo

5-fluorouracil (5FU) is an effective anti-cancer drug, yet its mechanism of action remains unclear. Here, we examine the effect of 5FU on pre-mRNA splicing in vivo. Using RT–PCR, we show that the splicing of a number of pre-mRNAs is inhibited in HeLa cells that have been exposed to a low dose of 5FU. It appears that this inhibitory effect is not due to its incorporation into pre-mRNA, because partially or fully 5FU-substituted pre-mRNA, when injected into Xenopus oocytes, is spliced just as well as is the unsubstituted pre-mRNA. Detailed analyses of 5FU-treated cells indicate that 5FU is incorporated into U2 snRNA at important naturally occurring pseudouridylation sites. Remarkably, 5FU incorporation effectively blocks the formation of important pseudouridines in U2 snRNA, as only a trace of pseudouridine is detected when cells are exposed to a low dose of 5FU for 5 days. Injection of the hypopseudouridylated HeLa U2 snRNA into U2-depleted Xenopus oocytes fails to reconstitute pre-mRNA splicing, whereas control U2 isolated from untreated or uracil-treated HeLa cells completely reconstitutes the splicing. Our results demonstrate for the first time that 5FU incorporates into a spliceosomal snRNA at natural pseudouridylation sites in vivo, thereby inhibiting snRNA pseudouridylation and splicing. This mechanism may contribute substantially to 5FU-mediated cell death.

Mechanistic investigations of the pseudouridine synthase RluA using RNA containing 5-fluorouridine

The pseuoduridine synthases (psi synthases) isomerize uridine (U) to pseudouridine (psi) in RNA, and they fall into five families that share very limited sequence similarity but have the same overall fold and active-site architecture, including an essential Asp. The mechanism by which the psi synthases operate remains unknown, and mechanistic work has largely made use of RNA containing 5-fluorouridine (f5U) in place of U. The psi synthase TruA forms a covalent adduct with such RNA, and heat disruption of the adduct generates a hydrated product of f5U, which was reasonably concluded to result from the hydrolysis of an ester linkage between the essential Asp and f5U. In contrast, the psi synthase TruB, which is a member of a different family, does not form an adduct with f5U in RNA but catalyzes the rearrangement and hydration of the f5U, which labeling studies with [18O]water showed does not result from ester hydrolysis. To extend the line of mechanistic investigation to another family of psi synthases and an enzyme that makes an adduct with f5U in RNA, the behavior of RluA toward RNA containing f5U was examined. Stem-loop RNAs are shown to be good substrates for RluA. Heat denaturation of the adduct between RluA and RNA containing f5U produces a hydrated nucleoside product, and labeling studies show that hydration does not occur by ester hydrolysis. These results are interpreted in light of a consistent mechanistic scheme for the handling of f5U by psi synthases.

A previously unidentified activity of yeast and mouse RNA:pseudouridine synthases 1 (Pus1p) on tRNAs

Mouse pseudouridine synthase 1 (mPus1p) was the first vertebrate RNA:pseudouridine synthase that was cloned and characterized biochemically. The mPus1p was previously found to catalyze Psi formation at positions 27, 28, 34, and 36 in in vitro produced yeast and human tRNAs. On the other hand, the homologous Saccharomyces cerevisiae scPus1p protein was shown to modify seven uridine residues in tRNAs (26, 27, 28, 34, 36, 65, and 67) and U44 in U2 snRNA. In this work, we expressed mPus1p in yeast cells lacking scPus1p and studied modification of U2 snRNA and several yeast tRNAs. Our data showed that, in these in vivo conditions, the mouse enzyme efficiently modifies yeast U2 snRNA at position 44 and tRNAs at positions 27, 28, 34, and 36. However, a tRNA:Psi26-synthase activity of mPus1p was not observed. Furthermore, we found that both scPus1p and mPus1p, in vivo and in vitro, have a previously unidentified activity at position 1 in cytoplasmic tRNAArg(ACG). This modification can take place in mature tRNA, as well as in pre-tRNAs with 5' and/or 3' extensions. Thus, we identified the protein carrying one of the last missing yeast tRNA:Psi synthase activities. In addition, our results reveal an additional activity of mPus1p at position 30 in tRNA that scPus1p does not possess.

Rapid tRNA decay can result from lack of nonessential modifications

The biological role of many nonessential tRNA modifications outside of the anticodon remains elusive despite their evolutionary conservation. We show here that m7G46 methyltransferase Trm8p/Trm82p acts as a hub of synthetic interactions with several tRNA modification enzymes, resulting in temperature-sensitive growth. Analysis of three double mutants indicates reduced levels of tRNA(Val(AAC)), consistent with a role of the corresponding modifications in maintenance of tRNA levels. Detailed examination of a trm8-delta trm4-delta double mutant demonstrates rapid degradation of preexisting tRNA(Val(AAC)) accompanied by its de-aminoacylation. Multiple copies of tRNA(Val(AAC)) suppress the trm8-delta trm4-delta growth defect, directly implicating this tRNA in the phenotype. These results define a rapid tRNA degradation (RTD) pathway that is independent of the TRF4/RRP6-dependent nuclear surveillance pathway. The degradation of an endogenous tRNA species at a rate typical of mRNA decay demonstrates a critical role of nonessential modifications for tRNA stability and cell survival.

A computational screen for mammalian pseudouridylation guide H/ACA RNAs

The box H/ACA RNA gene family is one of the largest non-protein-coding gene families in eukaryotes and archaea. Recently, we developed snoGPS, a computational screening program for H/ACA snoRNAs, and applied it to Saccharomyces cerevisiae. We report here results of extending our method to screen for H/ACA RNAs in multiple large genomes of related species, and apply it to the human, mouse, and rat genomes. Because of the 250-fold larger search space compared to S. cerevisiae, significant enhancements to our algorithms were required. Complementing extensive cloning experiments performed by others, our findings include the detection and experimental verification of seven new mammalian H/ACA RNAs and the prediction of 23 new H/ACA RNA pseudouridine guide assignments. These assignments include four for H/ACA RNAs previously classified as orphan H/ACA RNAs with no known targets. We also determined systematic syntenic conservation among human and mouse H/ACA RNAs. With this work, 82 of 97 ribosomal RNA pseudouridines and 18 of 32 spliceosomal RNA pseudouridines in mammals have been linked to H/ACA guide RNAs.

Biochemical and genetic analysis of the yeast proteome with a movable ORF collection

Functional analysis of the proteome is an essential part of genomic research. To facilitate different proteomic approaches, a MORF (moveable ORF) library of 5854 yeast expression plasmids was constructed, each expressing a sequence-verified ORF as a C-terminal ORF fusion protein, under regulated control. Analysis of 5573 MORFs demonstrates that nearly all verified ORFs are expressed, suggests the authenticity of 48 ORFs characterized as dubious, and implicates specific processes including cytoskeletal organization and transcriptional control in growth inhibition caused by overexpression. Global analysis of glycosylated proteins identifies 109 new confirmed N-linked and 345 candidate glycoproteins, nearly doubling the known yeast glycome.

Pseudouridine modification in Caenorhabditis elegans spliceosomal snRNAs: unique modifications are found in regions involved in snRNA-snRNA interactions

Background

Pseudouridine (Ψ) is an abundant modified nucleoside in RNA and a number of studies have shown that the presence of Ψ affects RNA structure and function. The positions of Ψ in spliceosomal small nuclear RNAs (snRNAs) have been determined for a number of species but not for the snRNAs from Caenorhabditis elegans (C. elegans), a popular experimental model system of development.

Results

As a prelude to determining the function of or requirement for this modification in snRNAs, we have mapped the positions of Ψ in U1, U2, U4, U5, and U6 snRNAs from worms using a specific primer extension method. As with other species, C. elegans U2 snRNA has the greatest number of Ψ residues, with nine, located in the 5' half of the U2 snRNA. U5 snRNA has three Ψs, in or near the loop of the large stem-loop that dominates the structure of this RNA. U6 and U1 snRNAs each have one Ψ, and two Ψ residues were found in U4 snRNA.

Conclusion

The total number of Ψs found in the snRNAs of C. elegans is significantly higher than the minimal amount found in yeasts but it is lower than that seen in sequenced vertebrate snRNAs. When the actual sites of modification on C. elegans snRNAs are compared with other sequenced snRNAs most of the positions correspond to modifications found in other species. However, two of the positions modified on C. elegans snRNAs are unique, one at position 28 on U2 snRNA and one at position 62 on U4 snRNA. Both of these modifications are in regions of these snRNAs that interact with U6 snRNA either in the spliceosome or in the U4/U6 small nuclear ribonucleoprotein particle (snRNP) and the presence of Ψ may be involved in strengthening the intermolecular association of the snRNAs.

Maintaining a conserved methylation in plant and insect U2 snRNA through compensatory mutation by nucleotide insertion

The extensive post-transcriptional modification of U2 snRNA is required for spliceosome assembly and pre-mRNA splicing in vertebrates. However, the rare modification of U2 snRNA in yeast implies a different mechanism for regulating spliceosome biogenesis in single-celled eukaryotes. To understand the evolutionary pattern of U2 snRNA methylation, we determined for the first time, the 2'-O-methylations of U2 snRNA in Oryza sativa, Arabidopsis thaliana and Drosophila melanogaster, and revealed two methylations which are conserved in a crucial region of U2 snRNA in plants. Interestingly, one of the methylations, U2-Cm29 is also methylated in D. melanogaster, but not in vertebrates. According to the methylation of U2-C29, computational analysis of databases identified three canonical box C/D snoRNAs, named OsmgU2-29, AtmgU2-29 and DmmgU2-28, as small methylation guides of U2 snRNA from O. sativa, A. thaliana and D. melanogaster, respectively. Although very divergent in their sequence, the three snoRNAs exhibit in common an 11 nucleotide-long sequence complementarity to corresponding U2 snRNA, implying a functional constraint on the modification during evolution. Interestingly, a nucleotide is found to be inserted both in U2 snRNA and DmmgU2-28 and maintains a perfect match of duplex specifying the methylation of C28 in Drosophila U2 snRNA. This is the first time a new model is being provided for compensatory mutations between a small guide RNA and its target by nucleotide insertion, instead of the known nucleotide substitution. In contrast to small Cajal body-specific RNAs (scaRNAs), the snoRNAs are similar to the reported singlet guide RNAs and are known to localize in nucleolus.

Elucidating the role of H/ACA-like RNAs in trans-splicing and rRNA processing via RNA interference silencing of the Trypanosoma brucei CBF5 pseudouridine synthase

Most pseudouridinylation in eukaryotic rRNA and small nuclear RNAs is guided by H/ACA small nucleolar RNAs. In this study, the Trypanosoma brucei pseudouridine synthase, Cbf5p, a snoRNP protein, was identified and silenced by RNAi. Depletion of this protein destabilized all small nucleolar RNAs of the H/ACA-like family. Following silencing, defects in rRNA processing, such as accumulation of precursors and inhibition of cleavages to generate the mature rRNA, were observed. snR30, an H/ACA RNA involved in rRNA maturation, was identified based on prototypical conserved domains characteristic of this RNA in other eukaryotes. The silencing of CBF5 also eliminated the spliced leader-associated (SLA1) RNA that directs pseudouridylation on the spliced leader RNA (SL RNA), which is the substrate for the trans-splicing reaction. Surprisingly, the depletion of Cbf5p not only eliminated the pseudouridine on the SL RNA but also abolished capping at the fourth cap-4 nucleotide. As a result of defects in the SL RNA and decreased modification on the U small nuclear RNA, trans-splicing was inhibited at the first step of the reaction, providing evidence for the essential role of H/ACA RNAs and the modifications they guide on trans-splicing.

NMR spectroscopy of RNA duplexes containing pseudouridine in supercooled water

We have performed NMR experiments in supercooled water in order to decrease the temperature-dependent exchange of protons in RNA duplexes. NMR spectra of aqueous samples of RNA in bundles of narrow capillaries that were acquired at temperatures as low as -18 degrees C reveal resonances of exchangeable protons not seen at higher temperatures. In particular, we detected the imino protons of terminal base pairs and the imino proton of a non-base-paired pseudouridine in a duplex representing the eukaryotic pre-mRNA branch site helix. Analysis of the temperature dependence of chemical shift changes (thermal coefficients) for imino protons corroborated hydrogen bonding patterns observed in the NMR-derived structural model of the branch site helix. The ability to observe non-base-paired imino protons of RNA is of significant value in structure determination of RNA motifs containing loop and bulge regions.

Pseudouridylation of yeast U2 snRNA is catalyzed by either an RNA-guided or RNA-independent mechanism

Yeast U2 small nuclear RNA (snRNA) contains three pseudouridines (Psi35, Psi42, and Psi44). Pus7p and Pus1p catalyze the formation of Psi35 and Psi44, respectively, but the mechanism of Psi42 formation remains unclear. Using a U2 substrate containing a single (32)P radiolabel at position 42, we screened a GST-ORF library for pseudouridylase activity. Surprisingly, we found a Psi42-specific pseudouridylase activity that coincided with Nhp2p, a protein component of a Box H/ACA sno/scaRNP (small nucleolar/Cajal body-specific ribonucleoprotein). When isolated by tandem affinity purification (TAP), the other protein components of the H/ACA sno/scaRNP also copurified with the pseudouridylase activity. Micrococcal nuclease-treated TAP preparations were devoid of pseudouridylase activity; however, activity was restored upon addition of RNAs from TAP preparations. Pseudouridylation reconstitution using RNAs from a Box H/ACA RNA library identified snR81, a snoRNA known to guide rRNA pseudouridylation, as the Psi42-specific guide RNA. Using the snR81-deletion strain, Nhp2p- or Cbf5p-conditional depletion strain, and a cbf5 mutation strain, we further demonstrated that the pseudouridylase activity is dependent on snR81 snoRNP in vivo. Our data indicate that snRNA pseudouridylation can be catalyzed by both RNA-dependent and RNA-independent mechanisms.

The many facets of H/ACA ribonucleoproteins

The H/ACA ribonucleoproteins (RNPs) are known as one of the two major classes of small nucleolar RNPs. They predominantly guide the site-directed pseudouridylation of target RNAs, such as ribosomal and spliceosomal small nuclear RNAs. In addition, they process ribosomal RNA and stabilize vertebrate telomerase RNA. Taken together, the function of H/ACA RNPs is essential for ribosome biogenesis, pre-mRNA splicing, and telomere maintenance. Every cell contains 100-200 different species of H/ACA RNPs, each consisting of the same four core proteins and one function-specifying H/ACA RNA. Most of these RNPs reside in nucleoli and Cajal bodies and mediate the isomerization of specific uridines to pseudouridines. Catalysis of the reaction is mediated by the putative pseudouridylase NAP57 (dyskerin, Cbf5p). Unexpectedly, mutations in this housekeeping enzyme are the major determinants of the inherited bone marrow failure syndrome dyskeratosis congenita. This review details the many diverse functions of H/ACA RNPs, some yet to be uncovered, with an emphasis on the role of the RNP proteins. The multiple functions of H/ACA RNPs appear to be reflected in the complex phenotype of dyskeratosis congenita.

A genome-wide analysis of C/D and H/ACA-like small nucleolar RNAs in Trypanosoma brucei reveals a trypanosome-specific pattern of rRNA modification

Small nucleolar RNAs (snoRNAs) constitute newly discovered noncoding small RNAs, most of which function in guiding modifications such as 2'-O-ribose methylation and pseudouridylation on rRNAs and snRNAs. To investigate the genome organization of Trypanosoma brucei snoRNAs and the pattern of rRNA modifications, we used a whole-genome approach to identify the repertoire of these guide RNAs. Twenty-one clusters encoding for 57 C/D snoRNAs and 34 H/ACA-like RNAs, which have the potential to direct 84 methylations and 32 pseudouridines, respectively, were identified. The number of 2'-O-methyls (Nms) identified on rRNA represent 80% of the expected modifications. The modifications guided by these RNAs suggest that trypanosomes contain many modifications and guide RNAs relative to their genome size. Interestingly, approximately 40% of the Nms are species-specific modifications that do not exist in yeast, humans, or plants, and 40% of the species-specific predicted modifications are located in unique positions outside the highly conserved domains. Although most of the guide RNAs were found in reiterated clusters, a few single-copy genes were identified. The large repertoire of modifications and guide RNAs in trypanosomes suggests that these modifications possibly play a central role in these parasites.

tRNA m7G methyltransferase Trm8p/Trm82p: evidence linking activity to a growth phenotype and implicating Trm82p in maintaining levels of active Trm8p

We show that Saccharomyces cerevisiae strains lacking Trm8p/Trm82p tRNA m7G methyltransferase are temperature-sensitive in synthetic media containing glycerol. Bacterial TRM8 orthologs complement the growth defect of trm8-Delta, trm82-Delta, and trm8-Delta trm82-Delta double mutants, suggesting that bacteria employ a single subunit for Trm8p/Trm82p function. The growth phenotype of trm8 mutants correlates with lack of tRNA m7G methyltransferase activity in vitro and in vivo, based on analysis of 10 mutant alleles of trm8 and bacterial orthologs, and suggests that m7G modification is the cellular function important for growth. Initial examination of the roles of the yeast subunits shows that Trm8p has most of the functions required to effect m7G modification, and that a major role of Trm82p is to maintain cellular levels of Trm8p. Trm8p efficiently cross-links to pre-tRNAPhe in vitro in the presence or absence of Trm82p, in addition to its known residual tRNA m7G modification activity and its SAM-binding domain. Surprisingly, the levels of Trm8p, but not its mRNA, are severely reduced in a trm82-Delta strain. Although Trm8p can be produced in the absence of Trm82p by deliberate overproduction, the resulting protein is inactive, suggesting that a second role of Trm82p is to stabilize Trm8p in an active conformation.

An early step in wobble uridine tRNA modification requires the Elongator complex

Elongator has been reported to be a histone acetyltransferase complex involved in elongation of RNA polymerase II transcription. In Saccharomyces cerevisiae, mutations in any of the six Elongator protein subunit (ELP1-ELP6) genes or the three killer toxin insensitivity (KTI11-KTI13) genes cause similar pleiotropic phenotypes. By analyzing modified nucleosides in individual tRNA species, we show that the ELP1-ELP6 and KTI11-KTI13 genes are all required for an early step in synthesis of 5-methoxycarbonylmethyl (mcm5) and 5-carbamoylmethyl (ncm5) groups present on uridines at the wobble position in tRNA. Transfer RNA immunoprecipitation experiments showed that the Elp1 and Elp3 proteins specifically coprecipitate a tRNA susceptible to formation of an mcm5 side chain, indicating a direct role of Elongator in tRNA modification. The presence of mcm5U, ncm5U, or derivatives thereof at the wobble position is required for accurate and efficient translation, suggesting that the phenotypes of elp1-elp6 and kti11-kti13 mutants could be caused by a translational defect. Accordingly, a deletion of any ELP1-ELP6 or KTI11-KTI13 gene prevents an ochre suppressor tRNA that normally contains mcm5U from reading ochre stop codons.

Detection and discovery of RNA modifications using microarrays

Using a microarray that tiles all known yeast non-coding RNAs, we compared RNA from wild-type cells with RNA from mutants encoding known and putative RNA modifying enzymes. We show that at least five types of RNA modification (dihydrouridine, m¹G, m22G, m¹A and m26A) catalyzed by 10 different enzymes (Trm1p, Trm5, Trm10p, Dus1p-Dus4p, Dim1p, Gcd10p and Gcd14p) can be detected by virtue of differential hybridization to oligonucleotides on the array that are complementary to the modified sites. Using this approach, we identified a previously undetected m¹A modification in GlnCTG tRNA, the formation of which is catalyzed by the Gcd10/Gcd14 complex.

The roles of the essential Asp-48 and highly conserved His-43 elucidated by the pH dependence of the pseudouridine synthase TruB

All known pseudouridine synthases have a conserved aspartic acid residue that is essential for catalysis, Asp-48 in Escherichia coli TruB. To probe the role of this residue, inactive D48C TruB was oxidized to generate the sulfinic acid cognate of aspartic acid. The oxidation restored significant but reduced catalytic activity, consistent with the proposed roles of Asp-48 as a nucleophile and general base. The family of pseudouridine synthases including TruB also has a nearly invariant histidine residue, His-43 in the E. coli enzyme. To examine the role of this conserved residue, site-directed mutagenesis was used to generate H43Q, H43N, H43A, H43G, and H43F TruB. Except for phenylalanine, the substitutions seriously impaired the enzyme, but all of the altered TruB retained significant activity. To examine the roles of Asp-48 and His-43 more fully, the pH dependences of wild-type, oxidized D48C, and H43A TruB were determined. The wild-type enzyme displays a typical bell-shaped profile. With oxidized D48C TruB, logk(cat) varies linearly with pH, suggesting the participation of specific rather than general base catalysis. Substitution of His-43 perturbs the pH profile, but it remains bell-shaped. The ascending limb of the pH profile is assigned to Asp-48, and the descending limb is tentatively ascribed to an active site tyrosine residue, the bound substrate uridine, or the bound product pseudouridine.

Psi35 in the branch site recognition region of U2 small nuclear RNA is important for pre-mRNA splicing in Saccharomyces cerevisiae

Pseudouridine 35 (psi35) in the branch site recognition region of yeast U2 small nuclear RNA is absolutely conserved in all eukaryotes examined. Pus7p catalyzes pseudouridylation at position 35 in Saccharomyces cerevisiae U2. The pus7 deletion strain, although viable in rich medium, is growth-disadvantaged under certain conditions. To clarify the function of U2 psi35 in yeast, we used this pus7 deletion strain to screen a collection of mutant U2 small nuclear RNAs, each containing a point mutation near the branch site recognition sequence, for a synthetic growth defect phenotype. The screen identified two U2 mutants, one containing a U40 --> G40 substitution (U40G) and another having a U40 deletion (U40Delta). Yeast strains carrying either of these U2 mutations grew as well as the wild-type strain in the selection medium, but they exhibited a temperature-sensitive growth defect phenotype when coupled with the pus7 deletion (pus7Delta). A subsequent temperature shift assay and a conditional pus7 depletion (via GAL promoter shutoff) in the U2-U40 mutant genetic background caused pre-mRNA accumulation, suggesting that psi35 is required for pre-mRNA splicing under certain conditions.

Pseudouridylation at position 32 of mitochondrial and cytoplasmic tRNAs requires two distinct enzymes in Saccharomyces cerevisiae

Cytoplasmic and mitochondrial tRNAs contain several pseudouridylation sites, and the tRNA:Psi-synthase acting at position 32 had not been identified in Saccharomyces cerevisiae. By combining genetic and biochemical analyses, we demonstrate that two enzymes, Rib2/Pus8p and Pus9p, are required for Psi32 formation in cytoplasmic and mitochondrial tRNAs, respectively. Pus9p acts mostly in mitochondria, and Rib2/Pus8p is strictly cytoplasmic. This is the first case reported so far of two distinct tRNA modification enzymes acting at the same position but present in two different compartments. This peculiarity may be the consequence of a gene fusion that occurred during yeast evolution. Indeed, Rib2/Pus8p displays two distinct catalytic activities involved in completely unrelated metabolism: its C-terminal domain has a DRAP-deaminase activity required for riboflavin biogenesis in the cytoplasm, whereas its N-terminal domain carries the tRNA:Psi32-synthase activity. Pus9p has only a tRNA:Psi32-synthase activity and contains a characteristic mitochondrial targeting sequence at its N terminus. These results are discussed in terms of RNA:Psi-synthase evolution.

Genome-wide searching for pseudouridylation guide snoRNAs: analysis of the Saccharomyces cerevisiae genome

One of the largest families of small RNAs in eukaryotes is the H/ACA small nucleolar RNAs (snoRNAs), most of which guide RNA pseudouridine formation. So far, an effective computational method specifically for identifying H/ACA snoRNA gene sequences has not been established. We have developed snoGPS, a program for computationally screening genomic sequences for H/ACA guide snoRNAs. The program implements a deterministic screening algorithm combined with a probabilistic model to score gene candidates. We report here the results of testing snoGPS on the budding yeast Saccharomyces cerevisiae. Six candidate snoRNAs were verified as novel RNA transcripts, and five of these were verified as guides for pseudouridine formation at specific sites in ribosomal RNA. We also predicted 14 new base-pairings between snoRNAs and known pseudouridine sites in S.cerevisiae rRNA, 12 of which were verified by gene disruption and loss of the cognate pseudouridine site. Our findings include the first prediction and verification of snoRNAs that guide pseudouridine modification at more than two sites. With this work, 41 of the 44 known pseudouridine modifications in S.cerevisiae rRNA have been linked with a verified snoRNA, providing the most complete accounting of the H/ACA snoRNAs that guide pseudouridylation in any species.

Crystal structure of the highly divergent pseudouridine synthase TruD reveals a circular permutation of a conserved fold

The pseudouridine (Psi) synthases Pus7p and TruD define a family of RNA-modifying enzymes with no sequence similarity to previously characterized Psi synthases. The 2.2 A resolution structure of Escherichia coli TruD reveals a U-shaped molecule with a catalytic domain that superimposes closely on that of other Psi synthases. A domain that appears to be unique to TruD/Pus7p family enzymes hinges over the catalytic domain, possibly serving to clasp the substrate RNAs. The active site comprises residues that are conserved in other Psi synthases, although at least one comes from a structurally distinct part of the protein. Remarkably, the connectivity of the structural elements of the TruD catalytic domain is a circular permutation of that of its paralogs. Because the sequence of the permuted segment, a beta-strand that bisects the catalytic domain, is conserved among orthologs from bacteria, archaea and eukarya, the permutation likely happened early in evolution.

X-ray structure of tRNA pseudouridine synthase TruD reveals an inserted domain with a novel fold

Pseudouridine synthases catalyse the isomerisation of uridine to pseudouridine in structural RNA. The pseudouridine synthase TruD, that modifies U13 in tRNA, belongs to a recently identified and large family of pseudouridine synthases present in all kingdoms of life. We report here the crystal structure of Escherichia coli TruD at 2.0 A resolution. The structure reveals an overall V-shaped molecule with an RNA-binding cleft formed between two domains: a catalytic domain and an insertion domain. The catalytic domain has a fold similar to that of the catalytic domains of previously characterised pseudouridine synthases, whereas the insertion domain displays a novel fold.

Detection and quantitation of RNA base modifications

Using a new combination of previously published techniques, we developed a method for quantitating modified nucleotides in RNAs. First, an RNA is cleaved with RNase H at the 5' side of a nucleotide of interest. Next, 32P is substituted for the phosphate at the 5' end of this nucleotide. Finally, after nuclease P1 digestion, the released radiolabeled nucleotide is analyzed by thin layer chromatography and quantitated by PhosphorImager. Using this method, we showed that the analysis of a pseudouridine at a specific site within an in vitro synthesized U2 RNA is indeed quantitative. We also applied this technique to cellular U2 RNA isolated from mouse liver, and showed that position U34 is approximately 90% pseudouridylated. This method, combined with previously described reverse transcription-based methods, constitutes a powerful tool for detecting and quantifying modified nucleotides in RNAs. With minor modifications, this method can serve as an effective assay to study RNA modifying enzymes.

Pseudouridines in and near the branch site recognition region of U2 snRNA are required for snRNP biogenesis and pre-mRNA splicing in Xenopus oocytes

Virtually all uridines in the branch site recognition region (BSRR) of vertebrate U2 are converted into pseudouridines after initial transcription. Here, we report a functional analysis of these modified nucleotides using the Xenopus oocyte reconstitution system. Using site-specific (32)P-labeling and TLC, we show that U2 pseudouridylation occurs much faster in the BSRR than in the 5'-terminal region. To functionally dissect the pseudouridines in the BSRR, we replaced each uridine with 5-fluorouridine (unmodifiable nucleotide) using site-specific RNase H cleavage directed by 2'-O-methyl-RNA-DNA chimeras followed by three-piece ligation. Whereas in vitro transcribed U2 containing no 5-fluorouridines rescued splicing in U2-depleted oocytes, no rescue was observed with U2 RNA containing 5-fluorouridines introduced into the BSRR. Additionally, U2 RNA containing 5-fluorouridines in the BSRR specifically inhibited pseudouridylation in the BSRR of in vitro transcribed U2 injected at a later time, although pseudouridylation in the 5'-end region was not affected. Our reconstitution results indicated that prior injection into U2-depleted oocytes with U2 RNA containing 5-fluorouridines in the BSRR almost completely abrogated the ability of in vitro transcribed U2 to rescue splicing, whereas full rescue was obtained with either cellular U2 or U2 containing pseudouridines in the BSRR. Further analyses using glycerol-gradient and native gel electrophoresis indicated that U2 RNAs lacking the BSRR pseudouridines do not participate in the assembly of the functionally active 17S U2 snRNP and the spliceosome. We conclude that the BSRR pseudouridines of vertebrate U2 are required for complete snRNP assembly and pre-mRNA splicing in Xenopus oocytes.

Not all pseudouridine synthases are potently inhibited by RNA containing 5-fluorouridine

RNA containing 5-fluorouridine has been assumed to inhibit strongly or irreversibly the pseudouridine synthases that act on the RNA. RNA transcripts containing 5-fluorouridine in place of uridine have, therefore, been added to reconstituted systems in order to investigate the importance of particular pseudouridine residues in a given RNA by inactivating the pseudouridine synthase responsible for their generation. In sharp contradiction to the assumption of universal inhibition of pseudouridine synthases by RNA containing 5-fluorouridine, the Escherichia coli pseudouridine synthase TruB, which has physiologically critical eukaryotic homologs, is not inhibited by such RNA. Instead, the RNA containing 5-fluorouridine was handled as a substrate by TruB. The E. coli pseudouridine synthase RluA, on the other hand, forms a covalent complex and is inhibited stoichiometrically by RNA containing 5-fluorouridine. We offer a hypothesis for this disparate behavior and urge caution in interpreting results from reconstitution experiments in which RNA containing 5-fluorouridine is assumed to inhibit a pseudouridine synthase, as normal function may result from a failure to inactivate the targeted enzyme rather than from the absence of nonessential pseudouridine residues.

The Saccharomyces cerevisiae U2 snRNA:pseudouridine-synthase Pus7p is a novel multisite-multisubstrate RNA:Psi-synthase also acting on tRNAs

The Saccharomyces cerevisiae Pus7 protein was recently characterized as a novel RNA:pseudouridine (Psi)-synthase acting at position 35 in U2 snRNA. However, U2 snRNA was the only potential substrate tested for this enzyme. In this work, we demonstrated that although Pus7p is responsible for the formation of only one of the six Psi residues present in yeast UsnRNAs, it catalyzes U to Psi conversion at position 13 in cytoplasmic tRNAs and at position 35 in pre-tRNA(Tyr). Sites of RNA modification by Pus7p were identified by analysis of the in vivo RNA modification defects resulting from the absence of active Pus7p production and by in vitro tests using extracts from WT and genetically modified yeast cells. For demonstration of the direct implication of Pus7p in RNA modification, the activity of the WT and mutated Pus7p recombinant proteins was tested on in vitro produced tRNA and pre-tRNA transcripts. Mutation of an aspartic acid residue (D256) that is conserved in all Pus7 homologs abolishes the enzymatic activity both in vivo and in vitro. This suggests the direct involvement of D256 in catalysis. Target sites of Pus7p in RNAs share a common sequence Pu(G/C)UNPsiAPu (Pu = purine, N = any nucleotide), which is expected to be important for substrate recognition. Modification of tRNAs by Pus7p explains the presence of Pus7p homologs in archaea and some bacteria species, which do not have U2 snRNA, and in vertebrates, where Psi34 (equivalent to Psi35 in yeast) formation in U2 snRNA is an H/ACA snoRNA guided process. Our results increase the number of known RNA modification enzymes acting on different types of cellular RNAs.

A novel unanticipated type of pseudouridine synthase with homologs in bacteria, archaea, and eukarya

Putative pseudouridine synthase genes are members of a class consisting of four subgroups that possess characteristic amino acid sequence motifs. These genes have been found in all organisms sequenced to date. In Escherichia coli, 10 such genes have been identified, and the 10 synthase gene products have been shown to function in making all of the pseudouridines found in tRNA and ribosomal RNA except for tRNA(Glu) pseudouridine13. In this work, a protein able to make this pseudouridine was purified by standard biochemical procedures. Amino-terminal sequencing of the isolated protein identified the synthase as YgbO. Deletion of the ygbO gene caused the loss of tRNA(Glu) pseudouridine13 and plasmid-borne restoration of the structural gene restored pseudouridine13. Reaction of the overexpressed gene product, renamed TruD, with a tRNA(Glu) transcript made in vitro also yielded only pseudouridine13. A search of the database detected 58 homologs of TruD spanning all three phylogenetic domains, including ancient organisms. Thus, we have identified a new wide-spread class of pseudouridine synthase with no sequence homology to the previously known four subgroups. The only completely conserved sequence motif in all 59 organisms that contained aspartate was GXKD, in motif II. This aspartate was essential for in vitro activity.