Plant nonsense suppressor tRNA(Tyr) genes are expressed at very low levels in vitro due to inefficient splicing of the intron-containing pre-tRNAs

Novel in vivo system to monitor tRNA expression based on the recovery of GFP fluorescence and its application for the determination of plant tRNA expression

We developed a novel assay system to quantitatively detect amber codon suppression by tRNAs expressed in plant cells. The assay was based on recovery of the expression of the green fluorescent protein (GFP) as a reporter, in which a fourth Lys codon (AAG) was changed to a premature amber codon TAG, designated as GFP/amber. Plasmids carrying GFP/amber, suppressor tRNA, and red fluorescent protein (RFF) as an internal control, respectively, were introduced into onion epidermal cells to monitor cell numbers with GFP and RFP fluorescence. First, an amber suppressor tRNA^Ser from tobacco (NtS2) to suppress a TAG codon in GFP mRNA was examined, leading to the recovery of GFP fluorescence. Second, we used two different tRNAs (i.e., AtY3II-am and AtY3II-amiG7), both of which are intron-containing amber suppressor tRNAs^Tyr, the former impaired precursor-tRNA splicing but the latter did not, as confirmed previously using two different approaches (Szeykowska-Kulinska and Beier, 1991; Akama and Beier, 2003). As expected, coexpression of GFP/amber with AtY3II-am gave no green fluorescence, but significant fluorescence was observed with AtY3II-amiG7. Then, we applied this system for the analysis of 5'-regulatory sequences of the tRNA^Gln gene family from Arabidopsis. A 5'-flanking sequence of each of the 17 tRNA^Gln genes was fused to a coding region of an amber suppressor tRNA^Ser gene (NtS2/amber) and its 3'-flanking sequence. Chimeric tRNA^Ser gene, GFP/amber, and RFP were coexpressed, and the GFP or RFP fluorescence intensity was determined in cells using laser-scanning microscopy. In parallel, 17 kinds of original Arabidopsis tRNA^Gln genes and their chimeric genes with NtS2/amber were all analyzed in cell-free nuclear extract (Yukawa et al., 1997). Comparison of in vitro and in vivo expression of these chimeric tRNA genes displayed generally similar results, accompanied by a wide range of variance in the expression of each gene. Nevertheless, the expression patterns of several genes were clearly the opposite of each other comparing between the two different system, demonstrating the importance of in vivo systems in the study on tRNA expression in plants.

Translational nonsense codon suppression as indicator for functional pre-tRNA splicing in transformed Arabidopsis hypocotyl-derived calli

The transient expression of three novel plant amber suppressors derived from a cloned Nicotiana tRNA(Ser)(CGA), an Arabidopsis intron-containing tRNA(Tyr)(GTA) and an Arabidopsis intron-containing tRNA(Met)(CAT) gene, respectively, was studied in a homologous plant system that utilized the Agro bacterium-mediated gene transfer to Arabidopsis hypocotyl explants. This versatile system allows the detection of beta-glucuronidase (GUS) activity by histochemical and enzymatic analyses. The activity of the suppressors was demonstrated by the ability to suppress a premature amber codon in a modified GUS gene. Co-transformation of Arabidopsis hypocotyls with the amber suppressor tRNA(Ser) gene and the GUS reporter gene resulted in approximately 10% of the GUS activity found in the same tissue transformed solely with the functional control GUS gene. Amber suppressor tRNAs derived from intron-containing tRNA(Tyr) or tRNA(Met) genes were functional in vivo only after some additional gene manipulations. The G3:C70 base pair in the acceptor stem of tRNA(Met)(CUA) had to be converted to a G3:U70 base pair, which is the major determinant for alanine tRNA identity. The inability of amber suppressor tRNA(Tyr) to show any activity in vivo predominantly results from a distorted intron secondary structure of the corresponding pre-tRNA that could be cured by a single nucleotide exchange in the intervening sequence. The improved amber suppressors tRNA(Tyr) and tRNA(Met) were subsequently employed for studying various aspects of the plant-specific mechanism of pre-tRNA splicing as well as for demonstrating the influence of intron-dependent base modifications on suppressor activity.

Identification of two catalytic subunits of tRNA splicing endonuclease from Arabidopsis thaliana

tRNA splicing endonuclease is essential for the correct removal of introns from precursor tRNA molecules of Archaea and Eucarya. The only well-characterized eucaryotic enzyme until now is the endonuclease from yeast (Saccharomyces cerevisiae). This protein has a heterotetrameric structure. Two of the four subunits, i.e. Sen34 and Sen44, contain the active sites for cleavage at the 3'- and 5'-splice sites, respectively. We have identified three novel genes from Arabidopsis thaliana, encoding putative subunits of tRNA splicing endonuclease. They are designated as AtSen1, AtSen2, and AtpsSen1. Both genes AtSen1 and AtSen2 seem to be functionally active, as deduced from corresponding cDNA sequences. Comparison of the amino acid sequences of the these two Arabidopsis proteins revealed 72% identity. However, AtpsSen1 is more similar to AtSen1, but is very likely a pseudogene, as concluded from extended stretches of deletions and the presence of in-frame stop codons. All putative proteins contain a conserved domain at their C-terminus common to counterparts from other organisms. Interestingly, they are more similar to the yeast catalytic subunit Sen44 than to Sen34. Southern analysis with various probes revealed that each gene is present as single copies in the nuclear genome. The evolutionary implications of these findings are discussed.

Splicing of arabidopsis tRNA(Met) precursors in tobacco cell and wheat germ extracts

Intron-containing tRNA genes are exceptional within nuclear plant genomes. It appears that merely two tRNA gene families coding for tRNA(GpsiA(Tyr)) and elongator tRNA(CmAU(Met)) contain intervening sequences. We have previously investigated the features required by wheat germ splicing endonuclease for efficient and accurate intron excision from Arabidopsis pre-tRNA(Tyr). Here we have studied the expression of an Arabidopsis elongator tRNA(Met) gene in two plant extracts of different origin. This gene was first transcribed either in HeLa or in tobacco cell nuclear extract and splicing of intron-containing tRNA(Met) precursors was then examined in wheat germ S23 extract and in the tobacco system. The results show that conversion of pre-tRNA(Met) to mature tRNA proceeds very efficiently in both plant extracts. In order to elucidate the potential role of specific nucleotides at the 3' and 5' splice sites and of a structured intron for pre-tRNA(Met) splicing in either extract, we have performed a systematic survey by mutational analyses. The results show that cytidine residues at intron-exon boundaries impair pre-tRNA(Met) splicing and that a highly structured intron is indispensable for pre-tRNA(Met) splicing. tRNA precursors with an extended anticodon stem of three to four base pairs are readily accepted as substrates by wheat and tobacco splicing endonuclease, whereas pre-tRNA molecules that can form an extended anticodon stem of only two putative base pairs are not spliced at all. An amber suppressor, generated from the intron-containing elongator tRNA(Met) gene, is efficiently processed and spliced in both plant extracts.

Characterization of nuclear tRNA(Tyr) introns: their evolution from red algae to higher plants

We have previously isolated numerous intron-containing nuclear tRNA(Tyr) genes derived from either monocotyledonous (Triticum) or dicotyledonous (Arabidopsis, Nicotiana) plants by screening the corresponding genomic phage libraries with a synthetic tRNA(Tyr)-specific oligonucleotide. Here we have characterized additional tRNA(Tyr) genes from phylogenetically divergent plant species representing red algae (Champia), brown algae (Cystophyllum), green algae (Ulva), stonewort (Chara), liverwort (Marchantia), moss (Polytrichum), fern (Rumohra) and gymnosperms (Ginkgo) using amplification of the coding sequences from the corresponding genomic DNAs by polymerase chain reaction (PCR). All novel tRNA(Tyr) genes contain intervening sequences of variable sequence and length ranging in size from 11 to 21 bp. However, two features are conserved in all plant pre-tRNA(Tyr) introns: they possess a uridine and less frequently an adenosine at the 5' boundary and can adopt similar intron secondary structures in which an extended anticodon helix of 4-5 bp is formed by base-pairing between nucleotides of the intron and the anticodon loop. In order to elucidate the potential role of the highly conserved uridine at the first intron position, we have replaced it by all other nucleosides in an Arabidopsis pre-tRNA(Tyr) and have studied in wheat germ extract its effect on splicing and on conversion of U to psi in the GpsiA anticodon. Furthermore, we discuss the putative acquisition of tRNA(Tyr) introns at an early step of evolution after the separation of Archaea and Eucarya.

Plant nuclear tRNA(Met) genes are ubiquitously interrupted by introns

We have isolated three independent clones for nuclear elongator tRNA(Met) genes from an Arabidopsis DNA library using a tRNA(Met)-specific probe generated by PCR. Each of the coding sequences for tRNAMet in these clones is identical and is interrupted by an identical 11 bp long intervening sequence at the same position in the anticodon loop of the tRNA. Their sequences differ at two positions from the intron in a soybean counterpart. Southern analysis of Arabidopsis DNA demonstrates that a gene family coding for tRNA(Met) is dispersed at at least eight loci in the genome. The unspliced precursor tRNA(Met) intermediate was detected by RNA analysis using an oligonucleotide probe complementary to the putative intron sequence. In order to know whether introns commonly interrupt plant tRNA(Met) genes, their coding sequences were PCR-amplified from the DNAs of eight phylogenetically separate plant species. All 53 sequences determined contain 10 to 13 bp long intervening sequences, always positioned one base downstream from the anticodon. They can all be potentially folded into the secondary structure characteristic for plant intron-containing precursor tRNAs. Surprisingly, GC residues are always present at the 5'-distal end of each intron.

The tRNATyr multigene family of Triticum aestivum: genome organization, sequence analyses and maturation of intron-containing pre-tRNAs in wheat germ extract

Southern analysis of Triticum DNA has revealed that nuclear tRNATyr genes are dispersed at a minimum of 16 loci in the genome. We have isolated six independent tRNATyr genes from a Triticum aestivum library in addition to three known members of the Triticum tRNATyr family. Four of the sequenced tRNATyr genes code for Triticum tRNA Tyr and two code for tRNA2Tyr. Three genes encode tRNAsTyr which carry one or two nucleotide substitutions as compared to the conventional genes. The nine Triticum tRNATyr genes possess highly conserved intron sequences ranging in size from 12 to 14 nucleotides. A common secondary intron structure with the 5' and 3' splice site loops separated by five base pairs can be formed by all pre-tRNAs Tyr which are efficiently spliced in the homologous wheat germ extract.

Mutations of Arabidopsis thaliana pre-tRNA(Tyr) affecting pseudouridylation of U35

The structural and sequence requirements for the biosynthesis of tRNA(Tyr) pseudouridine (psi 35) have been studied. Nucleotide substitution at the 32nd position slightly reduced modification efficiency in the case of transition (C32 to U32) while transversion (C32 to G32) had no effect on the modification process in wheat germ extract. Insertion of one nucleotide into the anticodon stem caused a 2-fold reduction of modification efficiency. Mutants with a partially deleted 12 nt long intron of pre-tRNA(Tyr) exhibited different effects: deletion of 5 nt (7 nt long intron) gave only a reduction in pseudouridylation while deletion of 7 nt (5 nt long intron) almost completely abolished the reaction. The generated mini-substrate consisting of pre-tRNA(Tyr) anticodon stem and intron sequence was partially modified which proved that the crucial elements for recognition of psi 35 introduction had to present in this construct.

The tRNA(Ser)-isoacceptors and their genes in Nicotiana rustica: genome organization, expression in vitro and sequence analyses

The existence of six serine codons results in a complex pattern of tRNA(Ser) isoacceptors in organisms and organelles. According to the original wobble hypothesis, a minimum of three isoacceptors should be sufficient to read the six serine codons. We have isolated five cytoplasmic tRNAs(Ser) from leaves of Nicotiana rustica. Their nucleotide sequences identify them as four different isoacceptors with the anticodons cm5UGA, CGA, IGA and GCU. For tRNA(Ser) with IGA anticodon, two species have been detected which vary only by one nucleotide in the long extra arm. The first three isoacceptors recognize codons of the type UCN whereas the fourth isoacceptor reads the two serine codons AGC and AGU. The tRNA(Ser) sequences were used to design appropriate primers for the amplification of Nicotiana nuclear tRNA(Ser) genes by the polymerase chain reaction (PCR). A total number of eight tRNA(Ser) genes differing in the coding region were thus identified. Selected PCR DNA fragments were then employed as probes for the isolation of the corresponding genes from a nuclear DNA library of N. rustica. Sequence analyses revealed that five of the isolated seven clones contained tRNA(Ser) genes which are identical in sequence with the five cytoplasmic tRNAs(Ser) mentioned above. None of them contains an intervening sequence. This is the first time that all putative cellular tRNA(Ser) isoacceptors and their corresponding genes have been characterized in an eukaryotic organism. Most of the tRNA(Ser) genes are functional as deduced from in vitro transcription and processing studies. Two of the genes yield pre-tRNAs(Ser) which are not or poorly converted to mature tRNA in a plant extract. The approximate tRNA(Ser) gene number was estimated by hybridization of specific DNA probes to Eco RI-cleaved Nicotiana nuclear DNA. The overall hybridization pattern indicates that members of each particular tRNA(Ser) gene family do not appear to be clustered but distributed randomly throughout the Nicotiana genome.

Transfer RNA-mediated suppression of stop codons in protoplasts and transgenic plants

We have developed a simple, rapid and sensitive assay for tRNA gene expression in plant cells. A plant tRNA(Leu) gene was site-specifically mutated to encode each of the three anticodon sequences (CUA, UUA and UCA) that recognize, respectively, the amber, ochre and opal stop codons. The suppression activity of these genes was detected by their ability to restore transient beta-glucuronidase (GUS) expression in tobacco protoplasts electroporated with GUS genes containing premature stop codons. Protoplasts co-electroporated with the amber suppressor tRNA gene and a GUS gene containing a premature amber stop codon showed up to 20-25% of the activity found in protoplasts transfected with the functional control GUS gene. Ochre and opal suppressors presented maximum efficiencies of less than 1%. This system could be adapted to examine transcription, processing or aminoacylation of tRNAs in plant cells. In addition, phenotypically normal, fertile tobacco plants expressing a stably incorporated amber suppressor tRNA gene have been obtained. This suppressor tRNA can be used to transactivate a target gene containing a premature amber stop codon by a factor of at least several hundred-fold.

The tRNA(Tyr) multigene family of Nicotiana rustica: genome organization, sequence analyses and expression in vitro

Tobacco tRNA(Tyr) genes are mainly organized as a dispersed multigene family as shown by hybridization with a tRNA(Tyr)-specific probe to Southern blots of Eco RI-digested DNA. A Nicotiana genomic library was prepared by Eco RI digestion of nuclear DNA, ligation of the fragments into the vector lambda gtWES.lambda B and in vitro packaging. The phage library was screened with a 5'-labelled synthetic oligonucleotide complementary to nucleotides 18 to 37 of cytoplasmic tobacco tRNA(Tyr). Eleven hybridizing Eco RI fragments ranging in size from 1.7 to 7.5 kb were isolated from recombinant lambda phage and subcloned into pUC19 plasmid. Four of the sequenced tRNA(Tyr) genes code for the known tobacco tRNA1(Tyr) (G psi A) and seven code for tRNA2(Tyr) (G psi A). The two tRNA species differ in one nucleotide pair at the basis of the T psi C stem. Only one tRNA(Tyr) gene (pNtY5) contains a point mutation (T54-->A54). Comparison of the intervening sequences reveals that they differ considerably in length and sequence. Maturation of intron-containing pre-tRNAs was studied in HeLa and wheat germ extracts. All pre-tRNAs(Tyr)--with one exception--are processed and spliced in both extracts. The tRNA(Tyr) gene encoded by pNtY5 is transcribed efficiently in HeLa extract but processing of the pre-tRNA is impaired.

The first human genes for tRNA(ArgICG), tRNA(GlyUCC), and tRNA(ThrIGU) and more tRNA(Val) pseudogenes: expression and pre-tRNA maturation in HeLa cell-free extracts

A functional tRNA(Val) gene, which codes for the major tRNA(ValIAC) isoacceptor species, and three new tRNA(Val) pseudogenes have been isolated from human genomic DNA. Two tRNA(Val) pseudogenes and a tRNA(Val) variant gene were found to be associated with tRNA genes encoding tRNA(ArgICG), tRNA(GlyUCC), and tRNA(ThrIGU), respectively, on distinct DNA fragments. All tRNA genes, including the pseudogenes, are actively transcribed in HeLa nuclear extract. Pre-tRNAs of tRNA(Val), tRNA(Arg), tRNA(Thr), and tRNA(Gly) genes are correctly processed to mature-sized tRNAs, whereas the three tRNA(Val) pseudogenes yield stable pre-tRNAs in vitro. These findings reveal that, together with the three known pseudogenes, half of the members of the human tRNA(Val) gene family are pseudogenes, all of which are active in homologous nuclear extracts in vitro and presumably also in vivo.

Intron excision from tRNA precursors by plant splicing endonuclease requires unique features of the mature tRNA domain

It has been proposed that yeast and Xenopus splicing endonucleases initially recognize features in the mature tRNA domain common to all tRNA species and that the sequence and structure of the intron are only minor determinants of splice-site selection. In accordance with this postulation, we show that yeast endonuclease splices heterologous pre-tRNA(Tyr) species from vertebrates and plants which differ in their mature domains and intron secondary structures. In contrast, wheat germ splicing endonuclease displays a pronounced preference for homologous pre-tRNA species; an extensive study of heterologous substrates revealed that neither yeast pre-tRNA species specific for leucine, serine, phenylalanine and tyrosine nor human and Xenopus pre-tRNA(Tyr) species were spliced. In order to identify the elements essential for pre-tRNA splicing in plants, we constructed chimeric genes coding for tRNA precursors with a plant intron secondary structure and with mature tRNA(Tyr) domains from yeast and Xenopus, respectively. The chimeric pre-tRNA comprising the mature tRNA(Tyr) domain from Xenopus was spliced efficiently in wheat germ extract, whereas the chimeric construct containing the mature tRNA(Tyr) domain from yeast was not spliced at all. These data indicate that intron secondary structure contributes to the specificity of plant splicing endonuclease and that unique features of the mature tRNA domain play a dominant role in enzyme-substrate recognition. We further investigated the influence of specific nucleotides in the mature domain on splicing by generating a number of mutated pre-tRNA species. Our results suggest that nucleotides located in the D stem, i.e. in the center of the pre-tRNA molecule, are recognition points for plant splicing endonuclease.

Expression of variant nuclear Arabidopsis tRNA(Ser) genes and pre-tRNA maturation differ in HeLa, yeast and wheat germ extracts

We have recently identified a tRNA gene cluster in the Arabidopsis nuclear genome. One tRNA(Ser) (AGA) gene and two tRNA(Tyr) (GTA) genes occur in tandem arrangement on a 1.5 kb unit that is amplified about 20-fold at a single chromosomal site. Here we have studied the in vitro expression of seven individually cloned tRNA(Ser) genes (pAtS1 to pAtS7) derived from this cluster. Five out of the seven tRNA(Ser) genes contain point mutations in the coding region which have in part adverse effects on the expression of these genes in different cell-free systems: (i) C10 and A62 in plant tRNA(Ser) genes, which correspond to G10 and C62, respectively, in all known vertebrate tRNA genes, result in a reduced transcription efficiency in HeLa but not in yeast extract. This indicates that yeast RNA polymerase III tolerates nucleotide substitutions at positions 10 [5' internal control region (ICR)] and 62 (3' ICR), whereas the vertebrate RNA polymerase III requires a more stringent consensus sequence. (ii) Processing of a pre-tRNA(Ser) with a mismatch in the aminoacyl stem is impaired in HeLa, yeast and wheat germ extracts, however, a mismatch in the anticodon stem is deleterious for HeLa and wheat germ but not for yeast processing enzymes. The unexpectedly high number of potential tRNA(Ser) pseudogenes in the cluster - quite in contrast to the tRNA(Ser) genes which mainly code for functional tRNAs - suggested that tRNA(Ser) (AGA) genes also occur elsewhere in the genome. We present evidence that single copies of tRNA(Ser) (AGA) genes do indeed exist outside the tRNA gene cluster.