Ac/Ds transposon mutagenesis in Arabidopsis thaliana: mutant spectrum and frequency of Ds insertion mutants

Molecular Lesions in BRI1 and Its Orthologs in the Plant Kingdom

Brassinosteroids (BRs) are an essential group of plant hormones regulating numerous aspects of plant growth, development, and stress responses. BRI1, along with its co-receptor BAK1, are involved in brassinosteroid sensing and early events in the BR signal transduction cascade. Mutational analysis of a particular gene is a powerful strategy for investigating its biochemical role. Molecular genetic studies, predominantly in Arabidopsis thaliana, but progressively in numerous other plants, have identified many mutants of the BRI1 gene and its orthologs to gain insight into its structure and function. So far, the plant kingdom has identified up to 40 bri1 alleles in Arabidopsis and up to 30 bri1 orthologs in different plants. These alleles exhibit phenotypes that are identical in terms of development and growth. Here, we have summarized bri1 alleles in Arabidopsis and its orthologs present in various plants including monocots and dicots. We have discussed the possible mechanism responsible for the specific allele. Finally, we have briefly debated the importance of these alleles in the research field and the agronomically valuable traits they offer to improve plant varieties.

Lotus Base: An integrated information portal for the model legume Lotus japonicus

Lotus japonicus is a well-characterized model legume widely used in the study of plant-microbe interactions. However, datasets from various Lotus studies are poorly integrated and lack interoperability. We recognize the need for a comprehensive repository that allows comprehensive and dynamic exploration of Lotus genomic and transcriptomic data. Equally important are user-friendly in-browser tools designed for data visualization and interpretation. Here, we present Lotus Base, which opens to the research community a large, established LORE1 insertion mutant population containing an excess of 120,000 lines, and serves the end-user tightly integrated data from Lotus, such as the reference genome, annotated proteins, and expression profiling data. We report the integration of expression data from the L. japonicus gene expression atlas project, and the development of tools to cluster and export such data, allowing users to construct, visualize, and annotate co-expression gene networks. Lotus Base takes advantage of modern advances in browser technology to deliver powerful data interpretation for biologists. Its modular construction and publicly available application programming interface enable developers to tap into the wealth of integrated Lotus data. Lotus Base is freely accessible at: https://lotus.au.dk.

Transcription profiling of fertilization and early seed development events in a solanaceous species using a 7.7 K cDNA microarray from Solanum chacoense ovules

BACKGROUND

To provide a broad analysis of gene expression changes in developing embryos from a solanaceous species, we produced amplicon-derived microarrays with 7741 ESTs isolated from Solanum chacoense ovules bearing embryos from all developmental stages. Our aims were to: 1) identify genes expressed in a tissue-specific and temporal-specific manner; 2) define clusters of genes showing similar patterns of spatial and temporal expression; and 3) identify stage-specific or transition-specific candidate genes for further functional genomic analyses.

RESULTS

We analyzed gene expression during S. chacoense embryogenesis in a series of experiments with probes derived from ovules isolated before and after fertilization (from 0 to 22 days after pollination), and from leaves, anthers, and styles. From the 6374 unigenes present in our array, 1024 genes were differentially expressed (>or= +/- 2 fold change, p value <or= 0.01) in fertilized ovules compared to unfertilized ovules and only limited expression overlap was observed between these genes and the genes expressed in the other tissues tested, with the vast majority of the fertilization-regulated genes specifically or predominantly expressed in ovules (955 genes). During embryogenesis three major expression profiles corresponding to early, middle and late stages of embryo development were identified. From the early and middle stages, a large number of genes corresponding to cell cycle, DNA processing, signal transduction, and transcriptional regulation were found. Defense and stress response-related genes were found in all stages of embryo development. Protein biosynthesis genes, genes coding for ribosomal proteins and other components of the translation machinery were highly expressed in embryos during the early stage. Genes for protein degradation were overrepresented later in the middle and late stages of embryo development. As expected, storage protein transcripts accumulated predominantly in the late stage of embryo development.

CONCLUSION

Our analysis provides the first study in a solanaceous species of the transcriptional program that takes place during the early phases of plant reproductive development, including all embryogenesis steps during a comprehensive time-course. Our comparative expression profiling strategy between fertilized and unfertilized ovules identified a subset of genes specifically or predominantly expressed in ovules while a closer analysis between each consecutive time point allowed the identification of a subset of stage-specific and transition-specific genes.

Large-scale phenotyping of transgenic tobacco plants (Nicotiana tabacum) to identify essential leaf functions

Two of the major challenges in functional genomics are to identify genes that play a key role in biological processes, and to elucidate the biological role of the large numbers of genes whose function is poorly characterized or still completely unknown. In this study, a combination of large-scale expressed sequence tag sequencing, high-throughput gene silencing and visual phenotyping was used to identify genes in which partial inhibition of expression leads to marked phenotypic changes, mostly on leaves. Three normalized tobacco (Nicotiana tabacum) cDNA libraries were prepared directly in a binary vector using different tissues of tobacco as an RNA source, randomly sequenced and clustered. The Agrobacterium-tobacco leaf disc transformation system was used to generate sets of antisense or co-suppression transgenic tobacco plants for over 20 000 randomly chosen clones, each representing an independent cluster. After transfer to the glasshouse, transgenic plants were scored visually after 10-14 days for changes in growth, leaf form and chlorosis or necrosis. Putative hits were validated by repeating the transformation. This procedure is more stringent than the analysis of knockout mutants, because it requires that even a partial decrease in expression generates a phenotype. This procedure identified 88 validated gene/phenotype relations. These included several previously characterized gene/phenotype relationships, demonstrating the validity of the approach. For about one-third, a function could be inferred, but a loss-of-function phenotype had not been described previously. Strikingly, almost one-half of the validated genes were poorly annotated, or had no known function. For 77 of these tobacco sequences, a single or small number of potential orthologues were identified in Arabidopsis. The genes for which orthologues were identified in Arabidopsis included about one-half of the genes whose function was completely unknown. Comparison with published gene/phenotype relations for Arabidopsis knockout mutants revealed surprisingly little overlap with the present study. Our results indicate that partial gene silencing identifies novel gene/phenotype relationships, which are distinct from those uncovered by knockout screens. They also show that it is possible to perform these analyses in a crop species in which full genome sequence information is lacking, and subsequently to transfer the information to a reference species in which functional studies can be performed more effectively.

The maize Activator/Dissociation system is functional in hexaploid wheat through successive generations

The aim of the present study was to provide useful background information and evidence of the functionality of the maize Activator/Dissociation (Ac/Ds) system in hexaploid wheat. Two transgenic parental wheat lines, one harbouring the immobilised Ac element (iAc) and the other the Ds element (pUbi[Ds-uidA]bar), were crossed. Transient GUS assays confirmed that the iAc transposase is active in hexaploid wheat. Selected F1 and F2 lines were analysed by PCR using primers specific to Ac, uidA and bar genes. The primer pair Ubi/bar-tag was used to detect excision of the Ds-uidA sequence, which occurred at a frequency of 39% in the F1 generation. Lines free of Ac and showing evidence of Ds excision were subject to Southern analysis, which indicated that at least one transposition event might have occurred in these lines. Although more evidence is required to unequivocally support the reintegration of the Ds element in the wheat genome, the evidence presented here nevertheless demonstrates the effectiveness and potential value of using this system to tag genes in wheat.

Chemical- and irradiation-induced mutants of indica rice IR64 for forward and reverse genetics

IR64, the most widely grown indica rice in South and Southeast Asia, possesses many positive agronomic characteristics (e.g., wide adaptability, high yield potential, tolerance to multiple diseases and pests, and good eating quality,) that make it an ideal genotype for identifying mutational changes in traits of agronomic importance. We have produced a large collection of chemical and irradiation-induced IR64 mutants with different genetic lesions that are amenable to both forward and reverse genetics. About 60,000 IR64 mutants have been generated by mutagenesis using chemicals (diepoxybutane and ethylmethanesulfonate) and irradiation (fast neutron and gamma ray). More than 38,000 independent lines have been advanced to M4 generation enabling evaluation of quantitative traits by replicated trials. Morphological variations at vegetative and reproductive stages, including plant architecture, growth habit, pigmentation and various physiological characters, are commonly observed in the four mutagenized populations. Conditional mutants such as gain or loss of resistance to blast, bacterial blight, and tungro disease have been identified at frequencies ranging from 0.01% to 0.1%. Results from pilot experiments indicate that the mutant collections are suitable for reverse genetics through PCR-detection of deletions and TILLING. Furthermore, deletions can be detected using oligomer chips suggesting a general technique to pinpoint deletions when genome-wide oligomer chips are broadly available. M4 mutant seeds are available for users for screening of altered response to multiple stresses. So far, more than 15,000 mutant lines have been distributed. To facilitate broad usage of the mutants, a mutant database has been constructed in the International Rice Information System (IRIS; http: //www.iris.irri.org) to document the phenotypes and gene function discovered by users.

Mapping of transposable element Dissociation inserts in Brassica oleracea following plant regeneration from streptomycin selection of callus

To investigate the potential of heterologous transposons as a gene-tagging system in broccoli (Brassica oleracea var. italica), we have introduced a Dissociation ( Ds)-based two-element transposon system. Ds has been cloned into a 35S-SPT excision-marker system, with transposition being driven by an independent 35S-transposase gene construct. In three successive selfed generations of plants, there was no evidence of germinal-excision events. In a previous study, we overcame this apparent inability to produce B. oleracea plants with germinal excisions by performing a novel tissue-culture technique to select for fully green shoots from seed with somatic excision events. The results showed a very high efficiency of regeneration of fully green plants (up to 65%), and molecular analysis showed that the plants contained the equivalent of a germinal-excision event. In this study, we followed the previous work by using inverse and nested PCR to generate probes of flanking genomic DNA adjacent to independently reinserted Ds elements, and these were hybridised to DNA from a double-haploid mapping population of B. oleracea. Seventeen Ds insertions and the original Ds T-DNA site have been localised, and these are spread over six (out of nine) linkage groups. Distribution of inserts show that 15 were found on a different linkage group to the original 'launch' site, and of these 11 were found to be clustered on two separate groups. Previous studies in other plant species have found that germinal excision of Ds predominantly moves to sites linked close to the donor site. However, this study shows a potential to produce plants with Ds insertion scattered over many unlinked sites.

Characterization of enhancer trap and gene trap harboring Ac/Ds transposon in transgenic rice

Insertion mutagenesis has become one of the most popular methods for gene functions analysis. Here we report a two-element Ac/Ds transposon system containing enhancer trap and gene trap for gene tagging in rice. The excision of Ds element was examined by PCR amplification. The excision frequency of Ds element varied from 0% to 40% among 20 F(2) populations derived from 11 different Ds parents. Southern blot analysis revealed that more than 70% of excised Ds elements reinserted into rice genome and above 70% of the reinserted Ds elements were located at different positions of the chromosome in rice. The result of histochemical GUS analysis indicated that 28% of enhancer trap and 22% of gene trap tagging plants displayed GUS activity in leaves, roots, flowers or seeds. The GUS positive lines will be useful for identifying gene function in rice.

Heterologous transposition in Ustilago maydis

The phytopathogenic basidiomycete Ustilago maydis has become a model system for the analysis of plant-pathogen interactions. The genome sequence of this organism will soon be available, increasing the need for techniques to analyse gene function on a broad basis. We describe a heterologous transposition system for U. maydis that is based on the Caenorhabditis transposon Tc1, which is known to function independently of host factors and to be active in evolutionarily distant species. We have established a nitrate reductase based two-component counterselection system to screen for Tc1 transposition. The element was shown to be functional and transposed to several different locations in the genome of U. maydis. The insertion pattern observed was consistent with the proposed general mechanism of Tc1/mariner integration and constitutes a proof of principle for the first heterologous transposition system in a basidiomycete species. By mapping the insertion site context to known genomic sequences, Tc1 insertion events were shown to occur on different chromosomes, but exhibit a preference for non-coding regions. Only 20% of the insertions were found in putative open reading frames. The establishment of this system will permit efficient gene tagging in U. maydis and possibly also in other fungi.

Brassinosteroid-regulated gene expression

Major brassinosteroid (BR) effects such as BR-induced growth are mediated through genomic pathways because RNA synthesis inhibitors and protein synthesis inhibitors interfere with these processes. A limited number of BR-regulated genes have been identified hitherto. The majority of genes (such as BRU1, CycD3, Lin6, OPR3, and TRIP-1) were identified by comparisons of BR-treated versus control-treated plants. However, altered transcript levels after BR application may not reflect normal physiological events. A complementary approach is the comparison of BR-deficient plants versus wild-type plants. No artificial treatments interfere with endogenous signaling pathways, but a subset of phenotypic alterations of phytohormone-deficient plants most probably is secondary. To identify genes that are subject to direct BR regulation, we analyzed CPD antisense and dwf1-6 (cbb1) mutant plants. Both show a mild phenotype in comparison with BR-deficient mutants such as cpd/cbb3, det2, and dwf4. Plants were grown under two different environments to filter out BR deficiency effects that occur only at certain environmental conditions. Finally, we established expression patterns after BR treatment of wild-type and dwf1-6 (cbb1) plants. Ideally, a BR-regulated gene displays a dose-response relationship in such a way that a gene with decreased transcript levels in BR-deficient plants is BR inducible and vice versa. Expression profile analysis of above ground part of plants was performed by means of Affymetrix Arabidopsis Genome Arrays.

RASPBERRY3 gene encodes a novel protein important for embryo development

We identified a new gene that is interrupted by T-DNA in an Arabidopsis embryo mutant called raspberry3. raspberry3 has "raspberry-like" cellular protuberances with an enlarged suspensor characteristic of other raspberry embryo mutants, and is arrested morphologically at the globular stage of embryo development. The predicted RASPBERRY3 protein has domains found in proteins present in prokaryotes and algae chloroplasts. Computer prediction analysis suggests that the RASPBERRY3protein may be localized in the chloroplast. Complementation analysis supports the possibility that the RASPBERRY3 protein may be involved in chloroplast development. Our experiments demonstrate the important role of the chloroplast, directly or indirectly, in embryo morphogenesis and development.

Toward understanding vesicle traffic and the guard cell model

Vesicular trafficking and membrane fusion are integral to cell growth and development with SNARE proteins, RabGTPases and their associates implicated in membrane fusion and secretion throughout the plant endomembrane system. Although the overall pattern of function is similar to that of animals and yeast, many aspects of endomembrane organization and vesicle trafficking appear unique to plants, for example, the control of cell and vacuolar expansion, asymmetric growth and cell division. However, the dominant membrane trafficking pathways have yet to be defined. Comparative genomics provide important information about vesicle trafficking elements but assigning biological roles based on sequence similarities is extremely difficult. Cellular and genetic approaches are reviewed here that have allowed visualization of vesicle trafficking in plants, including capacitance and dye methods, imaging and marker techniques, protein interactions and reverse genetics. Stomatal guard cells are discussed as cell models for identifying vesicle trafficking pathways and evidence points to a role for vesicle trafficking in stomatal function. For plants generally, kinetic analyses and biochemical studies suggest that several different pools of vesicles, and possibly different mechanisms for delivery, are available for vesicle traffic between endomembrane compartments and the plasma membrane. Characterizing these pathways, their functions and controls provides a major challenge for the future.

Insertional mutagenesis of genes required for seed development in Arabidopsis thaliana

The purpose of this project was to identify large numbers of Arabidopsis genes with essential functions during seed development. More than 120,000 T-DNA insertion lines were generated following Agrobacterium-mediated transformation. Transgenic plants were screened for defective seeds and putative mutants were subjected to detailed analysis in subsequent generations. Plasmid rescue and TAIL-PCR were used to recover plant sequences flanking insertion sites in tagged mutants. More than 4200 mutants with a wide range of seed phenotypes were identified. Over 1700 of these mutants were analyzed in detail. The 350 tagged embryo-defective (emb) mutants identified to date represent a significant advance toward saturation mutagenesis of EMB genes in Arabidopsis. Plant sequences adjacent to T-DNA borders in mutants with confirmed insertion sites were used to map genome locations and establish tentative identities for 167 EMB genes with diverse biological functions. The frequency of duplicate mutant alleles recovered is consistent with a relatively small number of essential (EMB) genes with nonredundant functions during seed development. Other functions critical to seed development in Arabidopsis may be protected from deleterious mutations by extensive genome duplications.

Arabidopsis genes essential for seedling viability: isolation of insertional mutants and molecular cloning

We have undertaken a large-scale genetic screen to identify genes with a seedling-lethal mutant phenotype. From screening approximately 38,000 insertional mutant lines, we identified >500 seedling-lethal mutants, completed cosegregation analysis of the insertion and the lethal phenotype for >200 mutants, molecularly characterized 54 mutants, and provided a detailed description for 22 of them. Most of the seedling-lethal mutants seem to affect chloroplast function because they display altered pigmentation and affect genes encoding proteins predicted to have chloroplast localization. Although a high level of functional redundancy in Arabidopsis might be expected because 65% of genes are members of gene families, we found that 41% of the essential genes found in this study are members of Arabidopsis gene families. In addition, we isolated several interesting classes of mutants and genes. We found three mutants in the recently discovered nonmevalonate isoprenoid biosynthetic pathway and mutants disrupting genes similar to Tic40 and tatC, which are likely to be involved in chloroplast protein translocation. Finally, we directly compared T-DNA and Ac/Ds transposon mutagenesis methods in Arabidopsis on a genome scale. In each population, we found only about one-third of the insertion mutations cosegregated with a mutant phenotype.

An efficient method for dispersing Ds elements in the barley genome as a tool for determining gene function

To devise a method for function-based gene isolation and characterization in barley, we created a plasmid containing the maize Activator (Ac) transposase (AcTPase) gene and a negative selection gene, codA, and a plasmid containing Dissociation (Ds) inverted-repeat ends surrounding the selectable herbicide resistance gene, bar. These plasmids were used to stably transform barley (Hordeum vulgare). In vitro assays, utilizing a Ds-interrupted uidA reporter gene, were used to demonstrate high-frequency excisions of Ds when the uidA construct was introduced transiently into stably transformed, AcTPase-expressing plant tissue. Crosses were made between stably transformed plants expressing functional transposase under the transcriptional control of either the putative AcTPase promoter or the promoter and first intron from the maize ubiquitin (Ubi1) gene, and plants containing Ds-Ubi-bar. In F(1) plants from these crosses, low somatic and germinal transposition frequencies were observed; however, in F(2) progeny derived from individual selfed F(1) plants, up to 47% of the plants showed evidence of Ds transposition. Further analyses of F(3) plants showed that approximately 75% of the transposed Ds elements reinserted into linked locations and 25% into unlinked locations. Transposed Ds elements in plants lacking the AcTPase transposase gene could be reactivated by reintroducing the transposase gene through classical genetic crossing, making this system functional for targeted gene tagging and studies of gene function. During the analysis of F(3) plants we observed two mutant phenotypes in which the transposed Ds elements co-segregate with the new phenotype, suggesting the additional utility of such a system for tagging genes.

Ac as a tool for the functional genomics of rice

To examine whether the maize autonomous transposable element Ac can be used for the functional analysis of the rice genome, we used Southern blot analysis to analyze the behaviour of Ac in 559 rice plants of four transgenic families through three successive generations. All families showed highly active transposition of Ac, and 103 plants (18.4%) contained newly transposed Ac insertions. In nine of the 12 independent transpositions analyzed, their germinal transmission was detected. Partial sequencing of 99 Ac-flanking sequences revealed that 21 clones exhibited significant similarities with protein-coding genes in databases and four of them matched rice cDNA sequences. These results indicate preferential Ac transposition into protein-coding rice genes. To examine the feasibility of PCR-based screening of gene knockouts in rice Ac plants, we prepared bulked genomic DNA from the leaves of approximately 6000 rice Ac plants and pooled the DNA according to a three-dimensional matrix. Of 14 randomly selected genes, two gene knockouts were identified, and one encoding a rice cytochrome P450 (CYP86) gene was shown to be stably inherited to the progeny. Together, these results suggest that Ac can be efficiently used for the functional analysis of the rice genome.

The Arabidopsis dwarf1 mutant is defective in the conversion of 24-methylenecholesterol to campesterol in brassinosteroid biosynthesis

Since the isolation and characterization of dwarf1-1 (dwf1-1) from a T-DNA insertion mutant population, phenotypically similar mutants, including deetiolated2 (det2), constitutive photomorphogenesis and dwarfism (cpd), brassinosteroid insensitive1 (bri1), and dwf4, have been reported to be defective in either the biosynthesis or the perception of brassinosteroids. We present further characterization of dwf1-1 and additional dwf1 alleles. Feeding tests with brassinosteroid-biosynthetic intermediates revealed that dwf1 can be rescued by 22alpha-hydroxycampesterol and downstream intermediates in the brassinosteroid pathway. Analysis of the endogenous levels of brassinosteroid intermediates showed that 24-methylenecholesterol in dwf1 accumulates to 12 times the level of the wild type, whereas the level of campesterol is greatly diminished, indicating that the defective step is in C-24 reduction. Furthermore, the deduced amino acid sequence of DWF1 shows significant similarity to a flavin adenine dinucleotide-binding domain conserved in various oxidoreductases, suggesting an enzymatic role for DWF1. In support of this, 7 of 10 dwf1 mutations directly affected the flavin adenine dinucleotide-binding domain. Our molecular characterization of dwf1 alleles, together with our biochemical data, suggest that the biosynthetic defect in dwf1 results in reduced synthesis of bioactive brassinosteroids, causing dwarfism.

Characterization of the transposition pattern of the Ac element in Arabidopsis thaliana using endonuclease I-SceI

We have investigated physical distances and directions of transposition of the maize transposable element Ac in Arabidopsis thaliana. We prepared a transferred DNA (T-DNA) construct that carried a non-autonomous derivative of Ac with a site for cleavage by endonuclease I-SceI (designated dAc-I-RS element). Another cleavage site was also introduced into the T-DNA region outside dAc-I-RS. Three transgenic Arabidopsis plants were generated, each of which had a single copy of the T-DNA at a different chromosomal location. These transgenic plants were crossed with the Arabidopsis that carried the gene for Ac transposase and progeny in which dAc-I-RS had been transposed were isolated. After digestion of the genomic DNA of these progeny with endonuclease I-SceI, sizes of segment of DNA were determined by pulse-field gel electrophoresis. We also performed linkage analysis for the transposed elements and sites of mutations near the elements. Our results showed that 50% of all transposition events had occurred within 1,700 kb on the same chromosome, with 35% within 200 kb, and that the elements transposed in both directions on the chromosome with roughly equal probability. The data thus indicate that the Ac-Ds system is most useful for tagging of genes that are present within 200 kb of the chromosomal site of Ac in Arabidopsis. In addition, determination of the precise localization of the transposed dAc-I-RS element should definitely assist in map-based cloning of genes around insertion sites.

Identification and isolation of the FEEBLY gene from tomato by transposon tagging

The Ac/Ds transposon system from maize was used for insertional mutagenesis in tomato. Marker genes were employed for the selection of plants carrying a total of 471 unique Ds elements. Three mutants were obtained with Ds insertions closely linked to recessive mutations: feebly (fb), yellow jim (yj) and dopey (dp). The fb seedlings produced high anthocyanin levels, developed into small fragile plants, and were insensitive to the herbicide phosphinothricin. The yj plants had yellow leaves as a result of reduced levels of chlorophyll. The dp mutants completely or partially lacked inflorescences. The fb and yj loci were genetically linked to the Ds donor site on chromosome 3. Reactivation of the Ds element in the fb mutants by crosses with an Ac-containing line resulted in restoration of the wild-type phenotypes. Plant DNA fragments flanking both sides of the Ds element in the fb mutant were isolated by the inverse polymerase chain reaction. Molecular analysis showed that phenotypic reversions of fb were correlated with excisions of Ds. DNA sequence analysis of Fb reversion alleles showed the characteristic Ds footprints. Northern and cDNA sequence analysis indicated that transcription of the FEEBLY (FB) gene was impeded by the insertion of Ds in an intron. Comparison of the predicted amino acid sequence of the FB protein with other database sequences indicated that FB is a novel gene.

Molecular analysis of the Arabidopsis pattern formation of gene GNOM: gene structure and intragenic complementation

The GNOM gene is required for pattern formation along the main body axis of the embryo in the flowering plant Arabidopsis thaliana. Mutations in the GNOM gene alter the asymmetric division of the zygote and interfere with the formation of distinct apical-basal regions in the developing embryo. We have isolated the GNOM gene by positional cloning, characterised its structure and determined the molecular lesions in mutant alleles. Although the predicted 163 kDa GNOM protein has a conserved domain in common with the yeast secretory protein Sec7p, it is most closely related in size and overall similarity to the product of the yeast YEC2 gene, which is not essential for cell viability. Four fully complementing gnom alleles carry missense mutations in conserved regions, seven partially complementing alleles have premature stop codon mutations and two non-complementing alleles have splice-site lesions. Our results suggest that the GNOM protein acts as a complex of identical subunits and that partial complementation may involve low levels of full-length protein generated by inefficient translational read-through.