In trypanosomatids, the majority of the guide (g) RNAs that provide the information for U-insertion/deletion RNA editing are encoded by minicircles that are catenated into large networks. In contrast, in the distantly related cryptobiid Trypanoplasma borreli, gRNA genes appear to reside in large 180-kb noncatenated DNA circles. To shed light on the evolutionary history and function of the minicircle network, we have analyzed minicircle organization in the free-living bodonid Bodo saltans, which is more closely related to trypanosomatids than T. borreli. We identified 1.4-kb circular DNAs as the B. saltans equivalent of minicircles via sequence analysis of 4 complete minicircles, 14 minicircle fragments, and 14 gRNAs. We show that each minicircle harbors two gRNA gene cassettes of opposite polarity residing in variable regions of about 200 nt in otherwise highly conserved molecules. In the conserved region, B. saltans minicircles contain a putative bent helix sequence and a degenerate dodecamer motif (CSB-3). Electron microscopy, sedimentation, and gel electrophoresis analyses showed no evidence for the existence of large minicircle networks in B. saltans, the large majority of the minicircles being present as circular and linear monomers (85–90%) with small amounts of catenated dimers and trimers. Our results provide the first example of a kinetoplastid species with noncatenated, gRNA gene-containing minicircles, which implies that the creation of minicircles and minicircle networks are separate evolutionary events.

We present a method to screen RNA for posttranscriptional modifications based on Matrix Assisted Laser Desorption/Ionization mass spectrometry (MALDI-MS). After the RNA is digested to completion with a nucleotide-specific RNase, the fragments are analyzed by mass spectrometry. A comparison of the observed mass data with the data predicted from the gene sequence identifies fragments harboring modified nucleotides. Fragments larger than dinucleotides were valuable for the identification of posttranscriptional modifications. A more refined mapping of RNA modifications can be obtained by using two RNases in parallel combined with further fragmentation by Post Source Decay (PSD). This approach allows fast and sensitive screening of a purified RNA for posttranscriptional modification, and has been applied on 5S rRNA from two thermophilic microorganisms, the bacterium Bacillus stearothermophilus and the archaeon Sulfolobus acidocaldarius, as well as the halophile archaea Halobacterium halobium and Haloarcula marismortui. One S. acidocaldarius posttranscriptional modification was identified and was further characterized by PSD as a methylation of cytidine32. The modified C is located in a region that is clearly conserved with respect to both sequence and position in B. stearothermophilus and H. halobium and to some degree also in H. marismortui. However, no analogous modification was identified in the latter three organisms. We further find that the 5′ end of H. halobium 5S rRNA is dephosphorylated, in contrast to the other 5S rRNA species investigated. The method additionally gives an immediate indication of whether the expected RNA sequence is in agreement with the observed fragment masses. Discrepancies with two of the published 5S rRNA sequences were identified and are reported here.

Introns in plant nuclear pre-mRNAs are highly enriched in U or U + A residues and this property is essential for efficient splicing. Moreover, 3′-untranslated regions (3′-UTRs) in plant pre-mRNAs are generally UA-rich and contain sequences that are important for the polyadenylation reaction. Here, we characterize two structurally related RNA-binding proteins (RBPs) from Nicotiana plumbaginifolia, referred to as RBP45 and RBP47, having specificity for oligouridylates. Both proteins contain three RBD-type RNA-binding domains and a glutamine-rich N-terminus, and share similarity with Nam8p, a protein associated with U1 snRNP in the yeast Saccharomyces cerevisiae. Deletion analysis of RBP45 and RBP47 indicated that the presence of at least two RBD are required for interaction with RNA and that domains other than RBD do not significantly contribute to binding. mRNAs for RBP45 and RBP47 and mRNAs encoding six related proteins in Arabidopsis thaliana are constitutively expressed in different plant organs. Indirect immunofluorescence and fractionation of cell extracts showed that RBP45 and RBP47 are localized in the nucleus. In vivo UV crosslinking experiments demonstrated their association with the nuclear poly(A)+ RNA. In contrast to UBP1, another oligouridylate-binding nuclear three-RBD protein of N. plumbaginifolia (Lambermon et al., EMBO J, 2000, 19:1638–1649), RBP45 and RBP47 do not stimulate mRNA splicing and accumulation when transiently overexpressed in protoplasts. Properties of RBP45 and RBP47 suggest they represent hnRNP-proteins participating in still undefined steps of pre-mRNA maturation in plant cell nuclei.

In eukaryotic cells, efficient translation of most cellular mRNAs requires the synergistic interplay between the m7GpppN cap structure and the poly(A) tail during initiation. We have developed and characterized a cell-free system from human HeLa cells that recapitulates this important feature, displaying more than one order of magnitude of translational synergism between the cap structure and the poly(A) tail. The stimulation of cap-dependent translation by the poly(A) tail is length-dependent, but not mediated by changes in mRNA stability. Using this system, we investigated the effect of the poly(A) tail on the translation of picornaviral RNAs, which are naturally polyadenylated but initiate translation via internal ribosome entry sites (IRESs). We show that translation driven by the IRESs of poliovirus (PV), encephalomyocarditis virus (EMCV), and hepatitis A virus is also significantly augmented by a poly(A) tail, ranging from an approximately 3-fold stimulation for the EMCV-IRES to a more than 10-fold effect for the PV IRES. These results raise interesting questions concerning the underlying molecular mechanism(s). The cell-free system described here should prove useful in studying these questions as well as providing a general biochemical tool to examine the translation initiation pathway in a more physiological setting.

The use of double-stranded RNA (dsRNA) to disrupt gene expression has become a powerful method of achieving RNA interference (RNAi) in a wide variety of organisms. However, in Trypanosoma brucei this tool is restricted to transient interference, because the dsRNA is not stably maintained and its effects are diminished and eventually lost during cellular division. Here, we show that genetic interference by dsRNA can be achieved in a heritable and inducible fashion. To show this, we established stable cell lines expressing dsRNA in the form of stem-loop structures under the control of a tetracycline-inducible promoter. Targeting α-tubulin and actin mRNA resulted in potent and specific mRNA degradation as previously observed in transient interference. Surprisingly, 10-fold down regulation of actin mRNA was not fatal to trypanosomes. This type of approach could be applied to study RNAi in other organisms that are difficult to microinject or electroporate. Furthermore, to quickly probe the consequences of RNAi for a given gene we established a highly efficient in vivo T7 RNA polymerase system for expression of dsRNA. Using the α-tubulin test system we obtained greater than 98% transfection efficiency and the RNAi response lasted at least two to three cell generations. These new developments make it possible to initiate the molecular dissection of RNAi both biochemically and genetically.

A cell-free mRNA decay assay has been adapted to permit the kinetics of 3′ → 5′ exoribonuclease activities to be monitored in real time. RNA probes containing 5′ caps and 3′ poly(A) tails generated by transcription in vitro are 3′ labeled using fluorescein-N6-ATP and poly(A) polymerase. Release of fluorescein-conjugated adenosine residues from the 3′ end of the RNA substrate is monitored by a time-dependent decrease in fluorescence anisotropy in the presence of cytosolic proteins. To demonstrate the utility of the assay, an RNA probe was constructed containing a fragment of the c-myc 3′ untranslated region and an 85-base poly(A) tail. Following 3′ fluorescein labeling, the rate of 3′-terminal adenosine excision was monitored in the presence of an S100 cytosolic extract prepared from K562 erythroleukemia cells. Removal of the fluorescein-tagged A residues resolved to a first-order decay function, allowing the rate constant and enzyme-specific activity to be determined in this extract. Further applications and advantages of this technology are discussed.

In eubacteria, base pairing between the 3′ end of 16S rRNA and the ribosome-binding site of mRNA is required for efficient initiation of translation. An interaction between the 18S rRNA and the mRNA was also proposed for translation initiation in eukaryotes. Here, we used an antisense RNA approach in vivo to identify the regions of 18S rRNA that might interact with the mRNA 5′ untranslated region (5′ UTR). Various fragments covering the entire mouse 18S rRNA gene were cloned 5′ of a cat reporter gene in a eukaryotic vector, and translation products were analyzed after transient expression in human cells. For the largest part of 18S rRNA, we show that the insertion of complementary fragments in the mRNA 5′ UTR do not impair translation of the downstream open reading frame (ORF). When translation inhibition is observed, reduction of the size of the complementary sequence to less than 200 nt alleviates the inhibitory effect. A single fragment complementary to the 18S rRNA 3′ domain retains its inhibitory potential when reduced to 100 nt. Deletion analyses show that two distinct sequences of approximately 25 nt separated by a spacer sequence of 50 nt are required for the inhibitory effect. Sucrose gradient fractionation of polysomes reveals that mRNAs containing the inhibitory sequences accumulate in the fractions with 40S ribosomal subunits, suggesting that translation is blocked due to stalling of initiation complexes. Our results support an mRNA–rRNA base pairing to explain the translation inhibition observed and suggest that this region of 18S rRNA is properly located for interacting with mRNA.

Telomeres in eukaryotic cells are generally synthesized and maintained by the ribonucleoprotein (RNP) telomerase. This enzyme is composed of at least two subunits, the telomerase reverse transcriptase (TERT) and the telomerase RNA. Human telomerase activity can be reconstituted in vitro by the expression of the telomerase protein catalytic subunit (hTERT) in the presence of recombinant human telomerase RNA (hTR) in a rabbit reticulocyte lysate (RRL) system. The hTERT and hTR subunits are independently expressed in vivo, and little is known about the mechanism of their assembly. To facilitate recombinant telomerase RNP formation and reconstitution, we engineered a construct, termed hTERT-hTR cis, in which the 3′ end of the hTERT coding sequence was extended by the addition of the sequence encoding hTR. Expression of the hTERT-hTR cis construct in vitro (in RRL) and in vivo (in the yeast Saccharomyces cerevisiae) produced hTERT-hTR transcripts of the predicted size. Active human telomerase was reconstituted by hTERT-hTR cis expression in both RRL and S. cerevisiae. Assembly of functional human telomerase by the bicistronic expression of the protein and RNA components may facilitate the overexpression and reconstitution of this enzyme in heterologous systems.

Nmd3p from yeast is required for the export of the large (60S) ribosomal subunit from the nucleus (Ho et al., 2000). Here, we show that Nmd3p forms a stable complex with free 60S subunits. Using an epitope-tagged Nmd3p, we show that free 60S subunits can be coimmunoprecipitated with Nmd3p. The interaction was specific for 60S subunits; 40S subunits were not coimmunoprecipitated. Using this coprecipitation technique and pulse-chase labeling of ribosomal subunit proteins we showed that Nmd3p bound nascent subunits, consistent with its role in export. However, under conditions in which ribosome biogenesis was inhibited (e.g., inhibition of transcription with thiolutin, inhibition of transcription of ribosomal protein and RNA genes in a sly1-1 mutant at nonpermissive temperature, and inhibition of translation in a conditional prt1 mutant), Nmd3p remained associated with 60S subunits. In addition, Nmd3Δ120, a truncated protein that lacked a nuclear localization signal, retained 60S binding. These results suggest that Nmd3p recruits nascent 60S subunits into the pool of free 60S subunits and exchanges on 60S subunits as they recycle during translation.

Three classes of RNA, represented by atpB and petD mRNAs, Arg and Glu tRNAs, and 5S rRNA, were found to exist in polyadenylated form in Chlamydomonas reinhardtii chloroplasts. Sequence analysis of cDNA clones derived from reverse transcriptase-polymerase chain reaction protocols used to select polyadenylated RNAs revealed that, at least for the mRNAs and tRNAs, there are three apparent types of polyadenylation. In the first case, the poly(A) tail is added at or near the mature 3′ end, even when this follows a strong secondary structure. In the second case, the tail is added to pre-mRNA or pre-tRNA, suggesting a possible competition between polyadenylation and RNA-processing pathways. Finally, in all cases, the poly(A) tail can be added internally, possibly as a part of an RNA-decay pathway. The tails found in Chlamydomonas chloroplasts differ from those of spinach chloroplasts in adenine content, being nearly homopolymeric (>98% adenine) versus 70% in spinach, and are similar in length to those of Escherichia coli, being mostly between 20 and 50 nt. In vitro assays using a Chlamydomonas chloroplast protein extract showed that a 3′ end A25 tail was sufficient to stimulate rapid degradation of atpB RNA in vitro, with a lesser effect for petD, and only minor effects on trnE. We therefore propose that polyadenylation contributes to mRNA degradation in Chlamydomonas chloroplasts, but that its effect may vary.

Messenger RNAs are exported from the nucleus as large ribonucleoprotein complexes (mRNPs). To date, proteins implicated in this process include TAP/Mex67p and RAE1/Gle2p and are distinct from the nuclear transport receptors of the β-related, Ran-binding protein family. Mex67p is essential for mRNA export in yeast. Its vertebrate homolog TAP has been implicated in the export of cellular mRNAs and of simian type D viral RNAs bearing the constitutive transport element (CTE). Here we show that TAP is predominantly localized in the nucleoplasm and at both the nucleoplasmic and cytoplasmic faces of the nuclear pore complex (NPC). TAP interacts with multiple components of the NPC including the nucleoporins CAN, Nup98, Nup153, p62, and with three major NPC subcomplexes. The nucleoporin-binding domain of TAP comprises residues 508–619. In HeLa cells, this domain is necessary and sufficient to target GFP-TAP fusions to the nuclear rim. Moreover, the isolated domain strongly competes multiple export pathways in vivo, probably by blocking binding sites on the NPC that are shared with other transport receptors. Microinjection experiments implicate this domain in the export of specific CTE-containing RNAs. Finally, we show that TAP interacts with transportin and with two proteins implicated in the export of cellular mRNAs: RAE1/hGle2 and E1B-AP5. The interaction of TAP with nucleoporins, its direct binding to the CTE RNA, and its association with two mRNP binding proteins suggest that TAP is an RNA export mediator that may bridge the interaction between specific RNP export substrates and the NPC.

Most eukaryotic mRNAs require the cap-binding complex eIF4F for efficient initiation of translation, which occurs as a result of ribosomal scanning from the capped 5′ end of the mRNA to the initiation codon. A few cellular and viral mRNAs are translated by a cap and end-independent mechanism known as internal ribosomal entry. The internal ribosome entry site (IRES) of classical swine fever virus (CSFV) is ∼330 nt long, highly structured, and mediates internal initiation of translation with no requirement for eIF4F by recruiting a ribosomal 43S preinitiation complex directly to the initiation codon. The key interaction in this process is the direct binding of ribosomal 40S subunits to the IRES to form a stable binary complex in which the initiation codon is positioned precisely in the ribosomal P site. Here, we report the results of analyses done using enzymatic footprinting and mutagenesis of the IRES to identify structural components in it responsible for precise binding of the ribosome. Residues flanking the initiation codon and extending from nt 363–391, a distance equivalent to the length of the 40S subunit mRNA-binding cleft, were strongly protected from RNase cleavage, as were nucleotides in the adjacent pseudoknot and in the more distal subdomain IIId1. Ribosomal binding and IRES-mediated initiation were abrogated by disruption of helix 1b of the pseudoknot and very severely reduced by mutation of the protected residues in IIId1 and by disruption of domain IIIa. These observations are consistent with a model for IRES function in which binding of the region flanking the initiation codon to the decoding region of the ribosome is determined by multiple additional interactions between the 40S subunit and the IRES.

Barley yellow dwarf virus RNA lacks both a 5′ cap and a poly(A) tail, yet it is translated efficiently. It contains a cap-independent translation element (TE), located in the 3′ UTR, that confers efficient translation initiation at the AUG closest to the 5′ end of the mRNA. We propose that the TE must both recruit ribosomes and facilitate 3′-5′ communication. To dissect its function, we determined the secondary structure of the TE and roles of domains within it. Nuclease probing and structure-directed mutagenesis revealed that the 105-nt TE (TE105) forms a cruciform secondary structure containing four helices connected by single-stranded regions. TE105 can function in either UTR in wheat germ translation extracts. A longer viral sequence (at most 869 nt) is required for full cap-independent translation in plant cells. However, substantial translation of uncapped mRNAs can be obtained in plant cells with TE105 combined with a poly(A) tail. All secondary structural elements and most primary sequences that were mutated are required for cap-independent translation in the 3′ and 5′ UTR contexts. A seven-base loop sequence was needed only in the 3′ UTR context. Thus, this loop sequence may be involved only in communication between the UTRs and not directly in recruiting translational machinery. This structural and functional analysis provides a framework for understanding an emerging class of cap-independent translation elements distinguished by their location in the 3′ UTR.

The replication-dependent histone mRNAs end in a conserved 26-nt sequence that forms a stem-loop structure. This sequence is required for histone pre-mRNA processing and plays a role in multiple aspects of histone mRNA metabolism. Two proteins that bind the 3′ end of histone mRNA are found in Xenopus oocytes. xSLBP1 is found in the nucleus, where it functions in histone pre-mRNA processing, and in the cytoplasm, where it may control histone mRNA translation and stability. xSLBP2 is a cytoplasmic protein, inactive in histone pre-mRNA processing, whose expression is restricted to oogenesis and early development. These proteins are similar only in their RNA-binding domains (RBD). A chimeric protein (1-2-1) in which the RBD of xSLBP1 has been replaced with the RBD of xSLBP2 binds the stem-loop with an affinity similar to the original protein. The 1-2-1 protein efficiently localizes to the nucleus of the frog oocyte, but is not active in processing of histone pre-mRNA in vivo. This protein does not support processing in a nuclear extract, but inhibits processing by competing with the active SLBP by binding to the substrate. The 1-2-1 protein also inhibits processing of synthetic histone pre-mRNA injected into frog oocytes, but has no effect on processing of histone pre-mRNA transcribed from an injected histone gene. This result suggests that sequences in the RBD of xSLBP1 give it preferential access to histone pre-mRNA transcribed in vivo.

To investigate the accuracy of a model [Giese et al., 1998, Biochemistry 37:1094–1100 and Mathews et al., 1999, J Mol Biol 288:911–940] that predicts the stability of RNA hairpin loops, optical melting studies were conducted on sets of hairpins previously determined to have unusually stable thermodynamic parameters. Included were the tetraloops GNRA and UNCG (where N is any nucleotide and R is a purine), hexaloops with UU first mismatches, and the hairpin loop of the iron responsive element, CAGUGC. The experimental values for the GNRA loops are in excellent agreement (ΔG°37 within 0.2 kcal/mol and melting temperature (TM) within 4 °C) with the values predicted by the model. When the UNCG hairpin loops are treated as tetraloops, and a bonus of 0.8 kcal/mol included in the prediction to account for the extra stable first mismatch (UG), the measured and predicted values are also in good agreement (ΔG°37 within 0.7 kcal/mol and TM within 3 °C). Six hairpins with unusually stable UU first mismatches also gave good agreement with the predictions (ΔG°37 within 0.5 kcal/mol and TM within 8 °C), except for hairpins closed by wobble base pairs. For these hairpins, exclusion of the additional stabilization term for UU first mismatches improved the prediction (ΔG°37 within 0.1 kcal/mol and TM within 3 °C). Hairpins with the iron-responsive element loop were not predicted well by the model, as measured ΔG°37 values were at least 1 kcal/mol greater than predicted.

The cleavage site of the Neurospora VS RNA ribozyme is located in a separate hairpin domain containing a hexanucleotide internal loop with an A-C mismatch and two adjacent G-A mismatches. The solution structure of the internal loop and helix Ia of the ribozyme substrate hairpin has been determined by nuclear magnetic resonance (NMR) spectroscopy. The 2 nt in the internal loop, flanking the cleavage site, a guanine and adenine, are involved in two sheared G.A base pairs similar to the magnesium ion-binding site of the hammerhead ribozyme. Adjacent to the tandem G.A base pairs, the adenine and cytidine, which are important for cleavage, form a noncanonical wobble A+-C base pair. The dynamic properties of the internal loop and details of the high-resolution structure support the view that the hairpin structure represents a ground state, which has to undergo a conformational change prior to cleavage. Results of chemical modification and mutagenesis data of the Neurospora VS RNA ribozyme can be explained in context with the present three-dimensional structure.

Bacterial ribosomal protein S7 initiates the folding of the 3′ major domain of 16S ribosomal RNA by binding to its lower half. The X-ray structure of protein S7 from thermophilic bacteria was recently solved and found to be a modular structure, consisting of an α-helical domain with a β-ribbon extension. To gain further insights into its interaction with rRNA, we cloned the S7 gene from Escherichia coli K12 into a pET expression vector and introduced 4 deletions and 12 amino acid substitutions in the protein sequence. The binding of each mutant to the lower half of the 3′ major domain of 16S rRNA was assessed by filtration on nitrocellulose membranes. Deletion of the N-terminal 17 residues or deletion of the β hairpins (residues 72–89) severely decreased S7 affinity for the rRNA. Truncation of the C-terminal portion (residues 138–178), which includes part of the terminal α-helix, significantly affected S7 binding, whereas a shorter truncation (residues 148–178) only marginally influenced its binding. Severe effects were also observed with several strategic point mutations located throughout the protein, including Q8A and F17G in the N-terminal region, and K35Q, G54S, K113Q, and M115G in loops connecting the α-helices. Our results are consistent with the occurrence of several sites of contact between S7 and the 16S rRNA, in line with its role in the folding of the 3′ major domain.

NaSSU1 is a complex nuclear group I intron found in several species of Naegleria, consisting of a large self-splicing group I ribozyme (NaGIR2), which itself is interrupted by a small, group I-like ribozyme (NaGIR1) and an open reading frame (ORF) coding for a homing endonuclease. The GIR1 ribozyme cleaves in vitro transcripts of NaSSU1 at two internal processing sites about 400 nt downstream of the 5′ end of the intron, proximal to the endonuclease ORF. Here we demonstrate that self-cleavage of the excised intron also occurs in vivo in Naegleria gruberi, generating an ORF-containing RNA that possesses a short leader with a sequence element likely to be involved in gene expression. To assess the functional significance of self-cleavage, we constructed a genetic system in Saccharomyces cerevisiae. First, a mutant yeast strain was selected with a mutation in all the rRNA genes, rendering the rDNA resistant to cleavage by the Naegleria endonuclease. Active endonuclease, which is otherwise lethal, could be expressed readily in these cells. Endonuclease activity also could be detected in extracts of yeast harboring plasmids in which the endonuclease ORF was embedded in its native context in the intron. Analysis of the RNA from these yeast cells showed that the excised intron RNA was processed as in N. gruberi. A mutant intron constructed to prevent self-cleavage of the RNA failed to express endonuclease activity. These results support the hypothesis that the NaGIR1-catalyzed self-cleavage of the intron RNA is a key event in expression of the endonuclease.

Pre-rRNA processing in eukaryotic cells requires participation of several snoRNPs. These include the highly conserved and abundant U3 snoRNP, which is essential for synthesis of 18S rRNA. Here we report the characterization of Rrp9p, a novel yeast U3 protein, identified via its homology to the human U3-55k protein. Epitope-tagged Rrp9p specifically precipitates U3 snoRNA, but Rrp9p is not required for the stable accumulation of this snoRNA. Genetic depletion of Rrp9p inhibits the early cleavages of the primary pre-rRNA transcript at A0, A1, and A2 and, consequently, production of 18S, but not 25S and 5.8S, rRNA. The hU3-55k protein can partially complement a yeast rrp9 null mutant, indicating that the function of this protein has been conserved. Immunoprecipitation of extracts from cells that coexpress epitope-tagged Rrp9p and various mutant forms of U3 snoRNA limits the region required for association of Rrp9p to the U3-specific box B/C motif. Box C is essential, whereas box B plays a supportive role.

In its natural context, the hairpin ribozyme is constructed around a four-way helical junction. This presents the two loops that interact to form the active site on adjacent arms, requiring rotation into an antiparallel structure to bring them into proximity. In the present study we have compared the folding of this form of the ribozyme and subspecies lacking either the loops or the helical junction using fluorescence resonance energy transfer. The complete ribozyme as a four-way junction folds into an antiparallel structure by the cooperative binding of magnesium ions, requiring 20–40 μM for half-maximal extent of folding ([Mg2+]1/2) and a Hill coefficient n = 2. The isolated junction (lacking the loops) also folds into a corresponding antiparallel structure, but does so noncooperatively (n = 1) at a higher magnesium ion concentration ([Mg2+]1/2 = 3 mM). Introduction of a G + 1A mutation into loop A of the ribozyme results in a species with very similar folding to the simple junction, and complete loss of ribozyme activity. Removal of the junction from the ribozyme, replacing it either with a strand break (serving as a hinge) or a GC5 bulge, results in greatly impaired folding, with [Mg2+]1/2 > 20 mM. The results indicate that the natural form of the ribozyme undergoes ion-induced folding by the cooperative formation of an antiparallel junction and loop–loop interaction to generate the active form of the ribozyme. The four-way junction thus provides a scaffold in the natural RNA that facilitates the folding of the ribozyme into the active form.