Analysis of the sequence of the mitochondrial cyt b gene of Teratocephalus lirellus revealed multiple frame shift events. As these sites coincided with homopolymeric tracks, sequencing was repeated and chromatograms were screened for poorly resolved peaks. No evidence for sequencing errors was found. We therefore reverse transcribed mRNA, using M-MLV reverse transcriptase and two primers for strongly conserved regions located anteriorly (CbF1: >5′-GTTTTGGAATGTTGGAAGGCTTTTAGG-3′) and posteriorly (CbR1: <5′-CCCAGATATCACCATAGCCAAATAGACG-3′) in the gene. The cDNA product was PCR amplified and sequenced, and compared with the gDNA sequences. This revealed insertions of a single U in eight sites in the mRNA sequence (Fig. 1).

We have identified a new pyrimidine-tract binding factor, PUF, that is required, together with U2AF, for efficient reconstitution of RNA splicing in vitro. The activity has been purified and consists of two proteins, PUF60 and the previously described splicing factor p54. p54 and PUF60 form a stable complex in vitro when cotranslated in a reaction mixture. PUF activity, in conjunction with U2AF, facilitates the association of U2 snRNP with the pre-mRNA. This reaction is dependent upon the presence of the large subunit of U2AF, U2AF65, but not the small subunit U2AF35. PUF60 is homologous to both U2AF65 and the yeast splicing factor Mud2p. The C-terminal domain of PUF60, the PUMP domain, is distantly related to the RNA-recognition motif domain, and is probably important in protein–protein interactions.

A series of HeLa cell lines which stably express β-globin pre-mRNAs carrying point mutations at nt 654, 705, or 745 of intron 2 has been developed. The mutations generate aberrant 5′ splice sites and activate a common 3′ cryptic splice site upstream leading to aberrantly spliced β-globin mRNA. Antisense oligonucleotides, which in vivo blocked aberrant splice sites and restored correct splicing of the pre-mRNA, revealed major differences in the sensitivity of these sites to antisense probes. Although the targeted pre-mRNAs differed only by single point mutations, the effective concentrations of the oligonucleotides required for correction of splicing varied up to 750-fold. The differences among the aberrant 5′ splice sites affected sensitivity of both the 5′ and 3′ splice sites; in particular, sensitivity of both splice sites was severely reduced by modification of the aberrant 5′ splice sites to the consensus sequence. These results suggest large differences in splicing of very similar pre-mRNAs in vivo. They also indicate that antisense oligonucleotides may provide useful tools for studying the interactions of splicing machinery with pre-mRNA.

Although the primary structures of class 1 polypeptide release factors (RF1 and RF2 in prokaryotes, eRF1 in eukaryotes) are known, the molecular basis by which they function in translational termination remains obscure. Because all class 1 RFs promote a stop-codon-dependent and ribosome-dependent hydrolysis of peptidyl-tRNAs, one may anticipate that this common function relies on a common structural motif(s). We have compared amino acid sequences of the available class 1 RFs and found a novel, common, unique, and strictly conserved GGQ motif that should be in a loop (coil) conformation as deduced by programs predicting protein secondary structure. Site-directed mutagenesis of the human eRF1 as a representative of class 1 RFs shows that substitution of both glycyl residues in this motif, G183 and G184, causes complete inactivation of the protein as a release factor toward all three stop codons, whereas two adjacent amino acid residues, G181 and R182, are functionally nonessential. Inactive human eRF1 mutants compete in release assays with wild-type eRF1 and strongly inhibit their release activity. Mutations of the glycyl residues in this motif do not affect another function, the ability of eRF1 together with the ribosome to induce GTPase activity of human eRF3, a class 2 RF. We assume that the novel highly conserved GGQ motif is implicated directly or indirectly in the activity of class 1 RFs in translation termination.

The inherent chemical instability of RNA under physiological conditions is primarily due to the spontaneous cleavage of phosphodiester linkages via intramolecular transesterification reactions. Although the protonation state of the nucleophilic 2′-hydroxyl group is a critical determinant of the rate of RNA cleavage, the precise geometry of the chemical groups that comprise each internucleotide linkage also has a significant impact on cleavage activity. Specifically, transesterification is expected to be proportional to the relative in-line character of the linkage. We have examined the rates of spontaneous cleavage of various RNAs for which the secondary and tertiary structures have previously been modeled using either NMR or X-ray crystallographic data. Rate constants determined for the spontaneous cleavage of different RNA linkages vary by almost 10,000-fold, most likely reflecting the contribution that secondary and tertiary structures make towards the overall chemical stability of RNA. Moreover, a correlation is observed between RNA cleavage rate and the relative in-line fitness of each internucleotide linkage. One linkage located within an ATP-binding RNA aptamer is predicted to adopt most closely the ideal conformation for in-line attack. This linkage has a rate constant for transesterification that is ∼12-fold greater than is observed for an unconstrained linkage and was found to be the most labile among a total of 136 different sites examined. The implications of this relationship for the chemical stability of RNA and for the mechanisms of nucleases and ribozymes are discussed.

The natural substrate cleaved by the hepatitis delta virus (HDV) ribozyme contains a 3′,5′-phosphodiester linkage at the cleavage site; however, a 2′,5′-linked ribose-phosphate backbone can also be cleaved by both trans-acting and self-cleaving forms of the HDV ribozyme. With substrates containing either linkage, the HDV ribozyme generated 2′,3′-cyclic phosphate and 5′-hydroxyl groups suggesting that the mechanisms of cleavage in both cases were by a nucleophilic attack on the phosphorus center by the adjacent hydroxyl group. Divalent metal ion was required for cleavage of either linkage. However, although the 3′,5′-linkage was cleaved slightly faster in Ca2+ than in Mg2+, the 2′,5′-linkage was cleaved in Mg2+ (or Mn2+) but not Ca2+. This dramatic difference in metal-ion specificity is strongly suggestive of a crucial metal-ion interaction at the active site. In contrast to the HDV ribozymes, cleavage at a 2′,5′-phosphodiester bond was not efficiently catalyzed by the hammerhead ribozyme. The relaxed linkage specificity of the HDV ribozymes may be due in part to lack of a rigid binding site for sequences 5′ to the cleavage site.

Sera of some patients afflicted with the inflammatory disease myositis contain antibodies of the anti-PL-12 type. A fraction of these polyclonal autoantibodies specifically precipitates the fully matured human tRNAAla bearing the anticodon IGC (PL-12 antigen). Earlier work (Bunn & Mathews, 1987, Science 238:116–119) had shown that the epitopes are located entirely within the anticodon stem-loop of the tRNAAla. Here we demonstrate that human anti-tRNAAla autoantibodies immunoprecipitate a synthetic polyribonucleotide containing inosine (I) and N1-methylinosine (m1I) separated by 2 nt as in the anticodon stem-loop of human tRNAAla. The shortest polyribonucleotide that can be immunoprecipitated corresponds to the pentanucleotide IpGpCpm1IpUp, which corresponds to part of the anticodon loop of human tRNAAla and lacks the stem-loop structure. The efficiency of immunoprecipitation was about four times greater with longer polyribonucleotides capable of forming a stem-loop structure, and was abolished by altering the relative positions of I and m1I within the synthetic polynucleotide. Synthetic oligodeoxyribonucleotide analogs of the tRNAAla stem-loop, containing the sequence dIpdGdCdm1Ip, are not antigenic. Our results show that human anti-tRNAAla autoantibodies selectively recognize chemical details of modified nucleotides (the 6-keto group of inosine-34 and the 6-keto group and the N1-methyl groups of N1-methylinosine-37) within an anticodon loop structure of a tRNA molecule. We also describe the chemical synthesis of the phosphoramidite derivatives corresponding to N1-methylinosine and N1-methyl-2′-deoxyinosine for use in the automatic chemical synthesis of oligonucleotides containing N1-methylinosine and N1-methyl-2′-deoxyinosine.

In the natural form of the hairpin ribozyme the two loop-carrying duplexes that comprise the majority of essential bases for activity form two adjacent helical arms of a four-way RNA junction. In the present work we have manipulated the sequence around the junction in a way known to perturb the global folding properties. We find that replacement of the junction by a different sequence that has the same conformational properties as the natural sequence gives closely similar reaction rate and Arrhenius activation energy for the substrate cleavage reaction. By comparison, rotation of the natural sequence in order to alter the three-dimensional folding of the ribozyme leads to a tenfold reduction in the kinetics of cleavage. Replacement with the U1 four-way junction that is resistant to rotation into the antiparallel structure required to allow interaction between the loops also gives a tenfold reduction in cleavage rate. The results indicate that the conformation of the junction has a major influence on the catalytic activity of the ribozyme. The results are all consistent with a role for the junction in the provision of a framework by which the loops are presented for interaction in order to create the active form of the ribozyme.

Splicing of U12-dependent introns requires the function of U11, U12, U6atac, U4atac, and U5 snRNAs. Recent studies have suggested that U6atac and U12 snRNAs interact extensively with each other, as well as with the pre-mRNA by Watson–Crick base pairing. The overall structure and many of the sequences are very similar to the highly conserved analogous regions of U6 and U2 snRNAs. We have identified the homologs of U6atac and U12 snRNAs in the plant Arabidopsis thaliana. These snRNAs are significantly diverged from human, showing overall identities of 65% for U6atac and 55% for U12 snRNA. However, there is almost complete conservation of the sequences and structures that are implicated in splicing. The sequence of plant U6atac snRNA shows complete conservation of the nucleotides that base pair to the 5′ splice site sequences of U12-dependent introns in human. The immediately adjacent AGAGA sequence, which is found in human U6atac and all U6 snRNAs, is also conserved. High conservation is also observed in the sequences of U6atac and U12 that are believed to base pair with each other. The intramolecular U6atac stem-loop structure immediately adjacent to the U12 interaction region differs from the human sequence in 9 out of 21 positions. Most of these differences are in base pairing regions with compensatory changes occurring across the stem. To show that this stem-loop was functional, it was transplanted into a human suppressor U6atac snRNA expression construct. This chimeric snRNA was inactive in vivo but could be rescued by coexpression of a U4atac snRNA expression construct containing compensatory mutations that restored base pairing to the chimeric U6atac snRNA. These data show that base pairing of U4atac snRNA to U6atac snRNA has a required role in vivo and that the plant U6atac intramolecular stem-loop is the functional analog of the human sequence.

In Drosophila, the spliceosomal protein SNF fulfills the functions of two vertebrate proteins, U1 snRNP-U1A and U2 snRNP-U2B″. The structure and sequence of SNF, U1A, and U2B″ are nearly identical with two RNA recognition motifs (RRM) separated by a short linker region, yet they have different RNA-binding properties: U1A binds U1 snRNA, U2B″ binds U2 snRNA, and SNF binds both snRNAs. Structure/function studies on the human proteins have identified motifs in the N-terminal RRM that are critical for RNA-binding specificity but have failed to identify a function for the C-terminal RRM. Interestingly, SNF is chimeric in these motifs, suggesting a basis for its dual specificity. Here, we test the importance of these motifs by introducing site-directed mutations in the snf coding region and examining the effects of these mutations on assembly into the snRNP and on snf function in vivo. We found that an N-terminal RRM mutant protein predicted to eliminate RNA binding still assembles into snRNPs and is capable of rescuing snf 's lethal phenotype only if the normally dispensable C-terminal RRM is present. We also found that the mixed motif in the “RNA-specificity” domain is necessary for SNF's dual function whereas the mixed motif in the U2A′-protein-binding region is not. Finally, we demonstrate that animals carrying a snf mutation that converts SNF from a bifunctional protein to a U1 snRNP-specific protein are viable. This unexpected result suggests that SNF's presence within the U2 snRNP is not essential for splicing.

Translation of UGA as selenocysteine requires specific RNA secondary structures in the mRNAs of selenoproteins. These elements differ in sequence, structure, and location in the mRNA, that is, coding versus 3′ untranslated region, in prokaryotes, eukaryotes, and archaea. Analyses of eukaryotic selenocysteine insertion sequence (SECIS) elements via computer folding programs, mutagenesis studies, and chemical and enzymatic probing has led to the derivation of a predicted consensus structural model for these elements. This model consists of a stem-loop or hairpin, with conserved nucleotides in the loop and in a non-Watson–Crick motif at the base of the stem. However, the sequences of a number of SECIS elements predict that they would diverge from the consensus structure in the loop region. Using site-directed mutagenesis to introduce mutations predicted to either disrupt or restore structure, or to manipulate loop size or stem length, we show that eukaryotic SECIS elements fall into two distinct classes, termed forms 1 and 2. Form 2 elements have additional secondary structures not present in form 1 elements. By either insertion or deletion of the sequences and structures distinguishing the two classes of elements while maintaining appropriate loop size, conversion of a form 1 element to a functional form 2-like element and of a form 2 to a functional form 1-like element was achieved. These results suggest commonality of function of the two classes. The information obtained regarding the existence of two classes of SECIS elements and the tolerances for manipulations of stem length and loop size should facilitate designing RNA molecules for obtaining high-resolution structural information about these elements.

This report describes a novel RNA-binding protein, SECp43, that associates specifically with mammalian selenocysteine tRNA (tRNASec). SECp43, identified from a degenerate PCR screen, is a highly conserved protein with two ribonucleoprotein-binding domains and a polar/acidic carboxy terminus. The protein and corresponding mRNA are generally expressed in rat tissues and mammalian cell lines. To gain insight into the biological role of SECp43, affinity-purified antibody was employed to identify its molecular partners. Surprisingly, the application of native HeLa cell extracts to a SECp43 antibody column results in the purification of a 90-nt RNA species identified by direct sequencing and Northern blot analysis as tRNASec. The purification of tRNASec by the antibody column is striking, based on the low abundance of this tRNA species. Using recombinant SECp43 as a probe for interacting protein partners, we also identify a 48-kDa interacting protein, which is a possible component of the mammalian selenocysteine insertion (SECIS) pathway. To our knowledge, SECp43 is the first cloned protein demonstrated to associate specifically with eukaryotic tRNASec.

We report a general method of measuring the complexity of SELEX pools. In analogy to measurements of genome size by C0t analysis, the complexity of a SELEX pool is measured by determining the reannealing rate of its double-stranded PCR product. We applied this technique to study the selection dynamics of a recently reported SELEX to neutrophil elastase. We found that the number of sequences decreased from 107 in round 6 to ∼60 by round 15, the final round. The intermediate rounds are a mixture of a high abundance/low complexity pool with a low abundance/high complexity pool. As the SELEX progresses, the former pool expands at the expense of the latter. This technique should be useful for studying and optimizing SELEX dynamics, as well as for monitoring the progress of SELEX experiments.

During splicing of nuclear pre-mRNAs, the first step liberates the 5′ exon (exon 1) and yields a lariat intron-3′exon (intron-exon 2) intermediate. The second step results in exon ligation. Previous results indicated that severe truncations of the 5′ exon of the actin pre-mRNA result in a block to the second splicing step in vitro in yeast extracts, leading to an accumulation of intron-exon 2 lariat intermediates. We show that exogenous exon 1 RNA oligonucleotides can chase these stalled intermediates into lariat intron and spliced exons. This reaction requires some of the cis elements and trans-acting factors that are required for a normal second step. There is no strong sequence requirement for the exon 1 added in trans, but oligonucleotides with complementarity to the U5 snRNA conserved loop perform the chase more efficiently. Using a dominant negative mutant of the DEAH-box ATPase Prp16p and ATP depletion, we show that the stalled intermediate is blocked after the Prp16p-dependent step. These results show that exogenous RNAs with various sequences but containing no splicing signals can be incorporated into spliceosomes and undergo RNA recombination and exon shuffling during the second step of pre-mRNA splicing.

The gene for RsuA, the pseudouridine synthase that converts U516 to pseudouridine in 16S ribosomal RNA of Escherichia coli, has been deleted in strains MG1655 and BL21/DE3. Deletion of this gene resulted in the specific loss of pseudouridine516 in both cell lines, and replacement of the gene in trans on a plasmid restored the pseudouridine. Therefore, rsuA is the only gene in E. coli with the ability to produce a protein capable of forming pseudouridine516. There was no effect on the growth rate of rsuA− MG1655 either in rich or minimal medium at either 24, 37, or 42 °C. Plasmid rescue of the BL21/DE3 rsuA− strain using pET15b containing an rsuA gene with aspartate102 replaced by asparagine or threonine demonstrated that neither mutant was active in vivo. This result supports a role for this aspartate, located in a unique GRLD sequence in this gene, at the catalytic center of the synthase. Induction of wild-type and the two mutant synthases in strain BL21/DE3 from genes in pET15b yielded a strong overexpression of all three proteins in approximately equal amounts showing that the mutations did not affect production of the protein in vivo and thus that the lack of activity was not due to a failure to produce a gene product. Aspartate102 is found in a conserved motif present in many pseudouridine synthases. The conservation and distribution of this motif in nature was assessed.

The large subunit of the mammalian U2AF heterodimer (U2AF65) is essential for splicing in vitro. To expand our understanding of how this protein functions in vivo, we have created a null allele of the gene encoding the Schizosaccharomyces pombe ortholog, U2AF59, and employed it in a variety of genetic complementation assays. First, analysis of an extensive series of double amino acid substitutions indicates that this splicing factor is surprisingly refractory to mutations. Second, despite extensive structural conservation, we find that metazoan large subunit orthologs cannot substitute in vivo for fission yeast U2AF59. Third, because the activity of U2AF65 in vitro involves binding to the 3′ polypyrimidine tract, we examined the splicing of introns containing or lacking this feature in a U2AF59 mutant described here as well as a previously isolated temperature-sensitive mutant (Potashkin et al., 1993, Science 262:573–575). Our data indicate that all four introns tested, including two that lack extensive runs of pyrimidines between the branchpoint and 3′ splice site, show splicing defects upon shifting to the nonpermissive condition. In all cases, splicing is blocked prior to the first transesterification reaction in the mutants, consistent with the role inferred for human U2AF65 based on in vitro experiments.

Escherichia coli tRNALysSUU, as well as human tRNALys3SUU, has 2-thiouridine derivatives at wobble position 34 (s2U* 34). Unlike the native tRNALysSUU, the full-length, unmodified transcript of human tRNALys3UUU and the unmodified tRNALys3UUU anticodon stem/loop (ASLLys3UUU) did not bind AAA- or AAG-programmed ribosomes. In contrast, the completely unmodified yeast tRNAPhe anticodon stem/loop (ASLPheGAA) had an affinity (Kd = 136 ± 49 nM) similar to that of native yeast tRNAPheGmAA (Kd = 103 ± 19 nM). We have found that the single, site-specific substitution of s2U34 for U34 to produce the modified ASLLysSUU was sufficient to restore ribosomal binding. The modified ASLLysSUU bound the ribosome with an affinity (Kd = 176 ± 62 nM) comparable to that of native tRNALysSUU (Kd = 70 ± 7 nM). Furthermore, in binding to the ribosome, the modified ASLLys3SUU produced the same 16S P-site tRNA footprint as did native E. coli tRNALysSUU, yeast tRNAPheGmAA, and the unmodified ASLPheGAA. The unmodified ASLLys3UUU had no footprint at all. Investigations of thermal stability and structure monitored by UV spectroscopy and NMR showed that the dynamic conformation of the loop of modified ASLLys3SUU was different from that of the unmodified ASLLysUUU, whereas the stems were isomorphous. Based on these and other data, we conclude that s2U34 in tRNALysSUU and in other s2U34-containing tRNAs is critical for generating an anticodon conformation that leads to effective codon interaction in all organisms. This is the first example of a single atom substitution (U34 → s2U34) that confers the property of ribosomal binding on an otherwise inactive tRNA.

The rat β-tropomyosin gene encodes two tissue-specific isoforms that contain the internal, mutually exclusive exons 6 (nonmuscle/smooth muscle) and 7 (skeletal muscle). We previously demonstrated that the 3′ splice site of exon 6 can be activated by introducing a 9-nt polyuridine tract at its 3′ splice site, or by strengthening the 5′ splice site to a U1 consensus binding site, or by joining exon 6 to the downstream common exon 8. Examination of sequences within exons 6 and 8 revealed the presence of two purine-rich motifs in exon 6 and three purine-rich motifs in exon 8 that could potentially represent exonic splicing enhancers (ESEs). In this report we carried out substitution mutagenesis of these elements and show that some of them play a critical role in the splice site usage of exon 6 in vitro and in vivo. Using UV crosslinking, we have identified SF2/ASF as one of the cellular factors that binds to these motifs. Furthermore, we show that substrates that have mutated ESEs are blocked prior to A-complex formation, supporting a role for SF2/ASF binding to the ESEs during the commitment step in splicing. Using pre-mRNA substrates containing exons 5 through 8, we show that the ESEs within exon 6 also play a role in cooperation between the 3′ and 5′ splice sites flanking this exon. The splicing of exon 6 to 8 (i.e., 5′ splice site usage of exon 6) was enhanced with pre-mRNAs containing either the polyuridine tract in the 3′ splice site or consensus sequence in the 5′ splice site around exon 6. We show that the ESEs in exon 6 are required for this effect. However, the ESEs are not required when both the polyuridine and consensus splice site sequences around exon 6 were present in the same pre-mRNA. These results support and extend the exon-definition hypothesis and demonstrate that sequences at the 3′ splice site can facilitate use of a downstream 5′ splice site. In addition, the data support the hypothesis that ESEs can compensate for weak splice sites, such as those found in alternatively spliced exons, thereby providing a target for regulation.

M1 RNA that contained 4′-thiouridine was photochemically cross-linked to different substrates and to a product of the reaction it governs. The locations of the cross-links in these photochemically induced complexes were identified. The cross-links indicated that different substrates share some contacts but have distinct binding modes to M1 RNA. The binding of some substrates also results in a substrate-dependent conformational change in the enzymatic RNA, as evidenced by the appearance of an M1 RNA intramolecular cross-link. The identification of the cross-links between M1 RNA and product indicate that they are shared with only one of the three cross-linked E-S complexes that were identified, an indication of noncompetitive inhibition by the product. We also examined whether the cross-linked complexes between M1 RNA and substrate(s) or product are altered in the presence of the enzyme's protein cofactor (C5 protein) and in the presence of different concentrations of divalent metal ions. C5 protein enhanced the yield of certain M1 RNA–substrate cross-linked complexes for both wild-type M1 RNA and a deletion mutant of M1 RNA (Δ[273–281]), but not for the M1 RNA–product complex. High concentrations of Mg2+ increased the yield of all M1 RNA–substrate complexes but not the M1 RNA–product complex.

The regulation of cauliflower mosaic virus (CaMV) pregenomic 35S RNA translation occurs via nonlinear ribosome migration (ribosome shunt) and is mediated by an elongated hairpin structure in the leader. The replacement of the viral leader by a series of short, low-energy stems in either orientation supports efficient ribosomal shunting, showing that the stem per se, and not its sequence, is recognized by the translation machinery. The requirement for cis-acting sequences from the unstructured terminal regions of the viral leader was analyzed: the 5′-terminal polypyrimidine stretch and the short upstream open reading frame (uORF) A stimulate translation, whereas the 3′-flanking region seems not to be essential. Based on these results, an artificial leader was designed with a stable stem flanked by unstructured sequences derived from parts of the 5′- and 3′-proximal regions of the CaMV 35S RNA leader. This artificial leader is shunt-competent in translation assays in vivo and in vitro, indicating that a low-energy stem, broadly used as a device to successfully interfere with ribosome scanning, can efficiently support translation, if preceded by a short uORF. The synthetic 140-nt leader can functionally replace the CaMV 35S RNA 600-nt leader, thus implicating the universal role that nonlinear ribosome scanning could play in translation initiation in eukaryotes.