No abstract

We have previously identified a functionally essential bulged stem-loop in the 3 untranslated region of the positive-stranded RNA genome of mouse hepatitis virus. This 68-nucleotide structure is composed of six stem segments interrupted by five bulges, and its structure, but not its primary sequence, is entirely conserved in the related bovine coronavirus. The functional importance of individual stem segments of this stem-loop was characterized by genetic analysis using targeted RNA recombination. We also examined the effects of stem segment mutations on the replication of mouse hepatitis virus defective interfering RNAs. These studies were complemented by enzymatic and chemical probing of the stem-loop. Taken together, our results confirmed most of the previously proposed structure, but they revealed that the terminal loop and an internal loop are larger than originally thought. Three of the stem segments were found to be essential for viral replication. Further, our results suggest that the stem segment at the base of the stem-loop is an alternative base-pairing structure for part of a downstream, and partially overlapping, RNA pseudoknot that has recently been shown to be necessary for bovine coronavirus replication.Mouse hepatitis virus (MHV), one of the best-characterized members of the coronavirus family, has a single-stranded, positive-sense RNA genome some 31 kb in length. Upon infection, the first two-thirds of this exceptional molecule is translated into an RNA-dependent RNA polymerase. Coronavirus RNA synthesis then proceeds by a unique and incompletely understood mechanism described by conflicting models (1, 16, 24, 44-46, 54, 55, 60). Initially, the genomic RNA becomes the template for, at the least, a full-length (negative-sense) antigenome. Further events produce a series of smaller, subgenomic RNAs of both polarities. The positive-sense subgenomic RNAs form a 3Ј nested set, with each containing a 70-nucleotide (nt) leader that is identical to the 5Ј end of the genome and is joined at a downstream site to a stretch of sequence identical to the 3Ј end of the genome. The negativesense subgenomic RNAs form a 5Ј nested set and are roughly 1/10 to 1/100 as abundant as their positive-sense counterparts, with each possessing the complement of this arrangement, including a 5Ј oligo(U) tract and a 3Ј antileader (10, 47).Many advances in investigating the mechanism of coronavirus RNA synthesis have been enabled by the discovery of defective interfering (DI) RNAs of MHV (29, 30, 52) and of other coronaviruses (5,34,39). DI RNAs are extensively deleted genomic remnants that replicate by using the RNA synthesis machinery of a helper virus, often interfering with viral genomic RNA replication. Studies of naturally occurring and artificially constructed DI RNAs, which can be transfected into helper virus-infected cells, have mapped cis-acting sequence elements from the genome that participate in replication and transcription. These deletion analyses have demonstrated that the minimal extent of the 3Ј terminus o...

Classical yeast genetics coupled with the cloning of regulatory genes by complementation of function is a powerful means of identifying and isolating trans-acting regulatory elements. One such regulatory gene is ADR1 which encodes a protein required for transcriptional activation of the glucose-repressible alcohol dehydrogenase (ADH2) gene. We now report the nucleotide sequence of ADR1; it encodes a polypeptide chain of 1,323 amino acids, of which the amino-terminal 302 amino acids are sufficient to stimulate ADH2 transcription. This active amino-terminal region shows amino-acid sequence homology with the repetitive DNA-binding domain of TFIIIA, an RNA polymerase III transcription factor of Xenopus laevis. Similar domains are found in proteins encoded at the Krüppel and Serendipity loci of Drosophila melanogaster. We discuss the implications of this structural homology and suggest that a similar domain may exist in other yeast regulatory proteins such as those encoded by GAL4 (ref. 13) and PPR1 (ref.14).

This manuscript provides nomenclature recommendations developed by an international workgroup to increase transparency and standardization of pharmacogenetic (PGx) result reporting. Presently, sequence variants identified by PGx tests are described using different nomenclature systems. In addition, PGx analysis may detect different sets of variants for each gene, which can affect interpretation of results. This practice has caused confusion and may thereby impede the adoption of clinical PGx testing. Standardization is critical to move PGx forward.

Early pre-rRNA processing events were examined in the ancient protozoan parasite Trypanosoma brucei and found to have both distinctive and conserved features. Two 5'-ETS cleavages occur: A' and the newly discovered A0. A' and A0 appear related to vertebrate and yeast primary pre-RNA cleavage sites, respectively. However, trypanosomatid primary rRNA transcripts can first be processed at the ITS1/5.8S boundary and 5'-ETS sequences then removed by consecutive cleavages at A', A0 and A1 at the 5'-ETS/SSU rRNA junction. 5'-ETS sequences previously crosslinked to U3 snoRNA were tested for their roles in rRNA processing using our new tagged rRNA system. Two distinct A'-adjacent sequence elements, which may pair with U3 hinge bases, were specifically required for SSU rRNA production, as was a downstream element. The latter element appears conserved with the yeast 5'-ETS U3 binding sequence, required for A0, A1 and A2 cleavages, in that they both share 10 bases complementary with U3 hinge sequences and lie upstream from A0 and A1 sites located in a potential stem-loop structure. The distinctive positioning of putative trypanosomatid U3 binding sites with respect to A" and A0 cleavages suggests that different U3-dependent mechanisms may direct each processing event.

First characterized in Trypanosoma brucei, the spliced leader-associated (SLA) RNA gene locus has now been isolated from the kinetoplastids Leishmania tarentolae and Trypanosoma cruzi. In addition to the T. brucei SLA RNA, both L. tarentolae and T. cruzi SLA RNA repeat units also yield RNAs of 75 or 76 nucleotides (nt), 92 or 94 nt, and ϳ450 or ϳ350 nt, respectively, each with significant sequence identity to transcripts previously described from the T. brucei SLA RNA locus. Cell fractionation studies localize the three additional RNAs to the nucleolus; the presence of box C/D-like elements in two of the transcripts suggests that they are members of a class of small nucleolar RNAs (snoRNAs) that guide modification and cleavage of rRNAs. Candidate rRNA-snoRNA interactions can be found for one domain in each of the C/D element-containing RNAs. The putative target site for the 75/76-nt RNA is a highly conserved portion of the small subunit rRNA that contains 2-O-ribose methylation at a conserved position (Gm1830) in L. tarentolae and in vertebrates. The 92/94-nt RNA has the potential to form base pairs near a conserved methylation site in the large subunit rRNA, which corresponds to position Gm4141 of small rRNA 2 in T. brucei. These data suggest that trypanosomatids do not obey the general 5-bp rule for snoRNA-mediated methylation.Posttranscriptional modifications to the rRNAs, including endonucleolytic cleavage, 2Ј-O-ribose methylation and pseudouridinylation, are mediated by small, nucleolar RNAs (sno RNAs). The most abundant snoRNA, U3, has been identified in numerous eukaryotes, including the kinetoplastids (27) and Euglena sp. (23). A requirement for U3 snoRNA in endonucleolytic cleavages of precursor rRNA transcripts has been established in yeast and vertebrate systems (31,36). Multiple other snoRNAs have been identified that are involved in the 2Ј-O-ribose methylation and pseudouridinylation of the 18S and 25/28S rRNAs (3,22,42,43), although the precise function of many snoRNAs remains unclear.Several criteria have been used to identify snoRNAs (24,36,50), including the presence of conserved sequence motifs, association with nucleolar proteins (fibrillarin, Gar1p, and Pop1p), complementarity to pre-rRNAs, and localization to the nucleolus. Like other cellular RNAs, snoRNAs are believed to exist as ribonucleoprotein complexes (36, 50) and have been divided into two major classes based on conserved sequence elements and protein associations. The box C/D snoRNAs usually contain box C and box D elements near their 5Ј and 3Ј ends, respectively; these elements are required for snoRNP interaction with the abundant nucleolar protein, fibrillarin. In vertebrates, box C/D snoRNAs are frequently processed from the introns of protein-encoding mRNAs (54), while in plants and yeasts they may be transcribed polycistronically (32, 50). The other class, the box H/ACA snoRNAs, contains two conserved sequence motifs: box H resides in a hinge region between two stem-loop structures, and the "ACA" trinucleotide motif resides 3 ...

U3 nucleolar small RNA (snRNA) is involved in early processing of the primary rRNA transcript. A secondary structure model for the unusually small Trypanosoma brucei U3 snRNA was deduced by comparative analysis of U3 snRNA sequences and by chemical modification and enzymatic cleavage of U3 snRNA in deproteinized and ribonucleoprotein (RNP) forms. Comprehensive alignment of U3 snRNAs from vertebrate, plant, fungal and protozoan species clearly delineated conserved and divergent features. The 5' domain of the T.brucei U3 snRNA appears to form one small, flexible 5' stem loop structure followed by a long single-stranded region; this model is a variation on 5' domain structures proposed for other U3 snRNAs which do not conform to a single model. The 3' domain of T.brucei U3 snRNA contains four single-stranded sequences conserved between U3 snRNAs. Of these, structural probing determined that the configurations of GAU region and box B and C sequences are altered by protein interactions in U3 snRNP. Conspicuously, the 3' domains of trypanosomal U3 snRNAs lack stem loops 11 and Ill, indicating that these structures are not required for conserved U3 snRNA functions.

RNA B is one of three abundant trimethylguanosine-capped U small nuclear RNAs (snRNAs) of Trypanosoma brucei which is not strongly identified with other U snRNAs by sequence homology. We show here that RNA B is a highly diverged U3 snRNA homolog likely involved in pre-rRNA processing. Sequence identity between RNA B and U3 snRNAs is limited; only two of four boxes of homology conserved between U3 snRNAs are obvious in RNA B. These are the box A homology, specific for U3 snRNAs, and the box C homology, common to nucleolar snRNAs and required for association with the nucleolar protein, fibrillarin. A 35-kDa T. brucei fibrillarin homolog was identified by using an anti-Physarum fibriliarin monoclonal antibody. RNA B and fibrillarin were localized in nucleolar fractions of the nucleus which contained pre-rRNAs and did not contain nucleoplasmic snRNAs. Fibriliarin and RNA B were precipitated by scleroderma patient serum S4, which reacts with fibrillarins from diverse organisms; RNA B was the only trimethylguanosine-capped RNA precipitated. Furthermore, RNA B sedimented with pre-rRNAs in nondenaturing sucrose gradients, similarly to U3 and other nucleolar snRNAs, suggesting that RNA B is hydrogen bonded to rRNA intermediates and might be involved in their processing.Two distinct classes of small nuclear RNAs (snRNAs) reside in the eukaryotic nucleus and are involved in processing and maturation of RNAs (5). One class, the nucleoplasmic snRNAs, includes five of the six most abundant snRNAs found in metazoan and yeast nuclei: Ul, U2, U4, U5, and U6 snRNAs. Small nuclear ribonucleoprotein (snRNP) particles contain the nucleoplasmic snRNAs, core proteins which include the conserved Sm antigens recognized by sera from lupus patients, and additional particle-specific proteins. Together with pre-mRNAs, these snRNP particles assemble into spliceosome complexes in which introns are removed and exons are spliced together. Other, less abundant nucleoplasmic snRNPs are also involved in aspects of mRNA biogenesis; the U7 snRNP is implicated in 3' end formation of histone pre-mRNA, and the Ull snRNP functions in polyadenylation.The second class of snRNAs encompasses the nucleolar snRNAs (12, 49) and contains U3, one of the six abundant nuclear snRNAs of metazoan and yeast nuclei. Nucleolar snRNAs are associated with a highly conserved nucleolar protein, fibrillarin, which is precipitated from an evolutionarily diverse range of organisms by autoimmune antibodies produced in scleroderma patients (21,28,32,41,42). The nucleolar snRNAs are believed to have roles in rRNA processing, since the nucleolus is the site of ribosome biogenesis and since nucleolar snRNAs hydrogen bond with pre-rRNAs (11,17,36,49