Plant tubulin genes: Structure and differential expression during development

Genome-wide identification and evolution of the tubulin gene family in Camelina sativa

BACKGROUND

Tubulins play crucial roles in numerous fundamental processes of plant development. In flowering plants, tubulins are grouped into α-, β- and γ-subfamilies, while α- and β-tubulins possess a large isotype diversity and gene number variations among different species. This circumstance leads to insufficient recognition of orthologous isotypes and significantly complicates extrapolation of obtained experimental results, and brings difficulties for the identification of particular tubulin isotype function. The aim of this research is to identify and characterize tubulins of an emerging biofuel crop Camelina sativa.

RESULTS

We report comprehensive identification and characterization of tubulin gene family in C. sativa, including analyses of exon-intron organization, duplicated genes comparison, proper isotype designation, phylogenetic analysis, and expression patterns in different tissues. 17 α-, 34 β- and 6 γ-tubulin genes were identified and assigned to a particular isotype. Recognition of orthologous tubulin isotypes was cross-referred, involving data of phylogeny, synteny analyses and genes allocation on reconstructed genomic blocks of Ancestral Crucifer Karyotype. An investigation of expression patterns of tubulin homeologs revealed the predominant role of N6 (A) and N7 (B) subgenomes in tubulin expression at various developmental stages, contrarily to general the dominance of transcripts of H7 (C) subgenome.

CONCLUSIONS

For the first time a complete set of tubulin gene family members was identified and characterized for allohexaploid C. sativa species. The study demonstrates the comprehensive approach of precise inferring gene orthology. The applied technique allowed not only identifying C. sativa tubulin orthologs in model Arabidopsis species and tracking tubulin gene evolution, but also uncovered that A. thaliana is missing orthologs for several particular isotypes of α- and β-tubulins.

Cotton microtubule-associated protein GhMAP20L5 mediates fiber elongation through the interaction with the tubulin GhTUB13

Targeting proteins for Xklp2 (TPX2s) comprise a class of MAPs that are essential for plant growth and development by regulating the dynamic changes of microtubules (MTs) and proper formation of cytoskeleton. However, the function of TPX2 proteins in cotton fiber development remains poorly understood. Here, we identified the function of a fiber elongation-specific TPX2 protein, GhMAP20L5, in cotton. Suppressed GhMAP20L5 gene expression in cotton (GhMAP20L5i) significantly reduced fiber elongation rate, fiber length and lint percentage. GhMAP20L5i fibers had thinner and looser secondary cell walls (SCW), and incompact helix twists. GhMAP20L5 specifically interacted with the tubulin GhTUB13 on the cytoskeleton. Gene coexpression analysis showed that GhMAP20L5 involved in multiple pathways related to cytoskeleton establishment and fiber cell wall formation and affected cellulase genes expressions. In summary, our results revealed that GhMAP20L5 is important for fiber development by regulating cytoskeleton establishment and the cellulose deposition in cotton.

Optimization of Genome Knock-In Method: Search for the Most Efficient Genome Regions for Transgene Expression in Plants

Plant expression systems are currently regarded as promising alternative platforms for the production of recombinant proteins, including the proteins for biopharmaceutical purposes. However, the accumulation level of a target protein in plant expression systems is still rather low compared with the other existing systems, namely, mammalian, yeast, and E. coli cells. To solve this problem, numerous methods and approaches have been designed and developed. At the same time, the random nature of the distribution of transgenes over the genome can lead to gene silencing, variability in the accumulation of recombinant protein, and also to various insertional mutations. The current research study considered inserting target genes into pre-selected regions of the plant genome (genomic “safe harbors”) using the CRISPR/Cas system. Regions of genes expressed constitutively and at a high transcriptional level in plant cells (housekeeping genes) that are of interest as attractive targets for the delivery of target genes were characterized. The results of the first attempts to deliver target genes to the regions of housekeeping genes are discussed. The approach of “euchromatization” of the transgene integration region using the modified dCas9 associated with transcription factors is considered. A number of the specific features in the spatial chromatin organization allowing individual genes to efficiently transcribe are discussed.

Genome-Wide Analysis of Tubulin Gene Family in Cassava and Expression of Family Member FtsZ2-1 during Various Stress

Filamentous temperature-sensitive protein Z (Tubulin/FtsZ) family is a group of conserved GTP-binding (guanine nucleotide-binding) proteins, which are closely related to plant tissue development and organ formation as the major component of the cytoskeleton. According to the published genome sequence information of cassava (Manihot esculenta Crantz), 23 tubulin genes (MeTubulins) were identified, which were divided into four main groups based on their type and phylogenetic characteristics. The same grouping generally has the same or similar motif composition and exon–intron structure. Collinear analysis showed that fragment repetition event is the main factor in amplification of cassava tubulin superfamily gene. The expression profiles of MeTubulin genes in various tissue were analyzed, and it was found that MeTubulins were mainly expressed in leaf, petiole, and stem, while FtsZ2-1 was highly expressed in storage root. The qRT-PCR results of the FtsZ2-1 gene under hormone and abiotic stresses showed that indole-3-acetic acid (IAA) and gibberellin A3 (GA3) stresses could significantly increase the expression of the FtsZ2-1 gene, thereby revealing the potential role of FtsZ2-1 in IAA and GA3 stress-induced responses.

Identification of TPX2 Gene Family in Upland Cotton and Its Functional Analysis in Cotton Fiber Development

Microtubules (MTs) are of importance to fiber development. The Xklp2 (TPX2) proteins as a class of microtubule-associated proteins (MAPs) play a key role in plant growth and development by regulating the dynamic changes of microtubules (MTs). However, the mechanism underlying this is unknown. The interactions between TPX2 proteins and tubulin protein, which are the main structural components, have not been studied in fiber development of upland cotton. Therefore, a genome-wide analysis of the TPX2 family was firstly performed in Gossypiumhirsutum L. This study identified 41 GhTPX2 sequences in the assembled G. hirsutum genome by a series of bioinformatic methods. Generally, this gene family is phylogenetically grouped into six subfamilies, and 41 G. hirsutum TPX2 genes (GhTPX2s) are distributed across 21 chromosomes. A heatmap of the TPX2 gene family showed that homologous GhTPX2 genes, GhWDLA2/7 and GhWDLA4/9, have large differences in expression levels between two upland cotton recombinant inbred lines (69307 and 69362) that are different in fiber quality at 15 and 20 days post anthesis. The relative data indicate that these four genes are down-regulated under oryzalin, which causes microtubule depolymerization, as determined via qRT-PCR. A subcellular localization experiment suggested that GhWDLA2 and GhWDLA7 are localized to the microtubule cytoskeleton, and GhWDLA4 and GhWDLA9 are only localized to the nucleus. However, only GhWDLA7 between GhWDLA2 and GhWDLA7 interacted with GhTUA2 in the yeast two-hybrid assay. These results lay the foundation for further function study of the TPX2 gene family.

Water-stress-induced inhibition of α-tubulin gene expression during growth, and its implications for reproductive success in rice

A drought-suppressed cDNA (RiP-3), encoding a putative α-tubulin protein was isolated from rice panicle at pollen-mother-cell meiosis stage. Analysis of its deduced amino acid sequence showed all the typical structural motifs for plant α-tubulins. The expression of α-tubulin transcripts was observed in all the reproductive organs of rice panicle, and in 5- or 15-day old seedlings, but not in mature leaves. Expression levels were positively correlated with the regions and periods of high growth, and the transcript level declined in parallel with drought-induced reduction in growth rates in all tissues examined. Immunoblot analysis of proteins separated by SDS-PAGE with anti-α-tubulin monoclonal antibody showed that the level of protein paralleled the changes in the transcript abundance in these organs. In situ immunolocalization of the α-tubulin protein in sections of the basal part of 5-day old seedlings showed that the highest levels of the protein were associated with the fastest growing leaf whorls, and the protein level declined upon a brief episode of water stress. Given the known critical role of tubulin in cell division and elongation, the results indicate that the expression of α-tubulin gene may be part of the events that suppress panicle elongation during water deficit, which is in turn a suspected cause of male reproductive failure and yield reduction in rice.

A plant natriuretic peptide-like molecule of the pathogen Xanthomonas axonopodis pv. citri causes rapid changes in the proteome of its citrus host

BACKGROUND

Plant natriuretic peptides (PNPs) belong to a novel class of peptidic signaling molecules that share some structural similarity to the N-terminal domain of expansins and affect physiological processes such as water and ion homeostasis at nano-molar concentrations. The citrus pathogen Xanthomonas axonopodis pv. citri possesses a PNP-like peptide (XacPNP) uniquely present in this bacteria. Previously we observed that the expression of XacPNP is induced upon infection and that lesions produced in leaves infected with a XacPNP deletion mutant were more necrotic and lead to earlier bacterial cell death, suggesting that the plant-like bacterial PNP enables the plant pathogen to modify host responses in order to create conditions favorable to its own survival.

RESULTS

Here we measured chlorophyll fluorescence parameters and water potential of citrus leaves infiltrated with recombinant purified XacPNP and demonstrate that the peptide improves the physiological conditions of the tissue. Importantly, the proteomic analysis revealed that these responses are mirrored by rapid changes in the host proteome that include the up-regulation of Rubisco activase, ATP synthase CF1 alpha subunit, maturase K, and alpha- and beta-tubulin.

CONCLUSIONS

We demonstrate that XacPNP induces changes in host photosynthesis at the level of protein expression and in photosynthetic efficiency in particular. Our findings suggest that the biotrophic pathogen can use the plant-like hormone to modulate the host cellular environment and in particular host metabolism and that such modulations weaken host defence.

Molecular cloning, expression profiling, and yeast complementation of 19 beta-tubulin cDNAs from developing cotton ovules

Microtubules are a major structural component of the cytoskeleton and participate in cell division, intracellular transport, and cell morphogenesis. In the present study, 795 cotton tubulin expressed sequence tags were analysed and 19 beta-tubulin genes (TUB) cloned from a cotton cDNA library. Among the group, 12 cotton TUBs (GhTUBs) are reported for the first time here. Transcription profiling revealed that nine GhTUBs were highly expressed in elongating fibre cells as compared with fuzzless-lintless mutant ovules. Treating cultured wild-type cotton ovules with exogenous phytohormones showed that individual genes can be induced by different agents. Gibberellin induced expression of GhTUB1 and GhTUB3, ethylene induced expression of GhTUB5, GhTUB9, and GhTUB12, brassinosteroids induced expression of GhTUB1, GhTUB3, GhTUB9, and GhTUB12, and lignoceric acid induced expression of GhTUB1, GhTUB3, and GhTUB12. When GhTUBs were transformed into the Saccharomyces cerevisiae inviable mutant, tub2, which is deficient in beta-tubulin, one ovule-specific and eight of nine fibre-preferential GhTUBs rescued this lethality. This study suggests that the proteins encoded by cotton GhTUBs are involved during cotton fibre development.

Differentially expression of Tua1, a tubulin-encoding gene, during flowering of tea plant Camellia sinensis (L.) O. Kuntze using cDNA amplified fragment length polymorphism technique

The complementary DNA (cDNA) amplified fragment length polymorphism technique was used to isolate transcript-derived fragments corresponding to genes involved in the flowering of tea plant. Comparative sequence analysis of an approximately 300 bp differential fragment amplified by primer combination E11M11 revealed 80%-84% similarity to the corresponding part of an a-tubulin gene of other species. The complete cDNA sequence of this a-tubulin was cloned by the rapid amplification of cDNA ends technique; its full length is 1537 bp and contains an open reading frame of 450 amino acid residues with two N-glycosylation sites and four protein kinase C phosphorylation sites. The deduced amino acid sequences did show significant homology to the a-tubulin from other plants that has been reported to be a pollen-specific protein and could be correlated with plant cytoplasm-nucleus-interacted male sterility. We named this complete cDNA Tua1. The nucleotide and amino acid sequence data of Tua1 have been recorded in the GenBank sequence database. This Tua1 gene was cloned into the pET-32a expression system and expressed in Escherichia coli BL21trxB(DE3). The molecular weight of expressed protein was deduced to be approximately 49 kDa. Western blot analysis was used to identify the temporal expression of Tua1 in tea plant. Further studies of the effect of Tua1 protein on pollen tube growth indicated the Tua1 solution obviously promoted the growth of tea pollen tube.

Disruption of microtubule organization and centrosome function by expression of tobacco mosaic virus movement protein

The movement protein (MP) of Tobacco mosaic virus mediates the cell-to-cell transport of viral RNA through plasmodesmata, cytoplasmic cell wall channels for direct cell-to-cell communication between adjacent cells. Previous in vivo studies demonstrated that the RNA transport function of the protein correlates with its association with microtubules, although the exact role of microtubules in the movement process remains unknown. Since the binding of MP to microtubules is conserved in transfected mammalian cells, we took advantage of available mammalian cell biology reagents and tools to further address the interaction in flat-growing and transparent COS-7 cells. We demonstrate that neither actin, nor endoplasmic reticulum (ER), nor dynein motor complexes are involved in the apparent alignment of MP with microtubules. Together with results of in vitro coprecipitation experiments, these findings indicate that MP binds microtubules directly. Unlike microtubules associated with neuronal MAP2c, MP-associated microtubules are resistant to disruption by microtubule-disrupting agents or cold, suggesting that MP is a specialized microtubule binding protein that forms unusually stable complexes with microtubules. MP-associated microtubules accumulate ER membranes, which is consistent with a proposed role for MP in the recruitment of membranes in infected plant cells and may suggest that microtubules are involved in this process. The ability of MP to interfere with centrosomal gamma-tubulin is independent of microtubule association with MP, does not involve the removal of other tested centrosomal markers, and correlates with inhibition of centrosomal microtubule nucleation activity. These observations suggest that the function of MP in viral movement may involve interaction with the microtubule-nucleating machinery.

Molecular characterization of the cotton GhTUB1 gene that is preferentially expressed in fiber

Each fiber of cotton (Gossypium hirsutum) is a single epidermal cell that rapidly elongates to 2.5 to 3.0 cm from the ovule surface within about 16 d after anthesis. A large number of genes are required for fiber differentiation and development, but so far, little is known about how these genes control and regulate the process of fiber development. To investigate gene expression patterns in fiber, a cDNA, GhTUB1, encoding beta-tubulin was isolated from a cotton fiber cDNA library. The analyses of RNA northern-blot hybridization and reverse transcriptase-polymerase chain reaction demonstrated that GhTUB1 transcripts preferentially accumulated at high levels in fiber, at low levels in ovules at the early stage of cotton boll development, and at very low levels in other tissues of cotton. The corresponding GhTUB1 gene including the promoter region was isolated by screening a cotton genomic DNA library. To demonstrate the specificity of the GhTUB1 promoter, the 5'-flanking region including the promoter and 5'-untranslated region was fused with the beta-glucuronidase reporter gene. The expression of the reporter chimera was examined in a large number of transgenic cotton plants. Histochemical assays demonstrated that GhTUB1::beta-glucuronidase fusion genes were expressed preferentially at high levels in fiber and primary root tip of 1- to 3-d-old seedlings and at low levels in other tissues such as ovule, pollen, seedling cotyledon, and root basal portion. The results suggested that the GhTUB1 gene may play a distinct and required role in fiber development. In addition, the GhTUB1 promoter may have great potential for cotton improvement by genetic engineering.

Spatiotemporal relationships between growth and microtubule orientation as revealed in living root cells of Arabidopsis thaliana transformed with green-fluorescent-protein gene construct GFP-MBD

Arabidopsis thaliana plants were transformed with GFP-MBD (J. Marc et al., Plant Cell 10: 1927-1939, 1998) under the control of a constitutive (35S) or copper-inducible promoter. GFP-specific fluorescence distributions, levels, and persistence were determined and found to vary with age, tissue type, transgenic line, and individual plant. With the exception of an increased frequency of abnormal roots of 35S GFP-MBD plants grown on kanamycin-containing media, expression of GFP-MBD does not appear to affect plant phenotype. The number of leaves, branches, bolts, and siliques as well as overall height, leaf size, and seed set are similar between wild-type and transgenic plants as is the rate of root growth. Thus, we conclude that the transgenic plants can serve as a living model system in which the dynamic behavior of microtubules can be visualized. Confocal microscopy was used to simultaneously monitor growth and microtubule behavior within individual cells as they passed through the elongation zone of the Arabidopsis root. Generally, microtubules reoriented from transverse to oblique or longitudinal orientations as growth declined. Microtubule reorientation initiated at the ends of the cell did not necessarily occur simultaneously in adjacent neighboring cells and did not involve complete disintegration and repolymerization of microtubule arrays. Although growth rates correlated with microtubule reorientation, the two processes were not tightly coupled in terms of their temporal relationships, suggesting that other factor(s) may be involved in regulating both events. Additionally, microtubule orientation was more defined in cells whose growth was accelerating and less stringent in cells whose growth was decelerating, indicating that microtubule-orienting factor(s) may be sensitive to growth acceleration, rather than growth per se.

Gene-specific changes in alpha-tubulin transcript accumulation in developing cotton fibers

The fibers of cotton (Gossypium hirsutum) are single-cell trichomes that undergo rapid and synchronous elongation. Cortical microtubules provide spatial information necessary for the alignment of cellulose microfibrils that confine and regulate cell elongation. We used gene-specific probes to investigate alpha-tubulin transcript levels in elongating cotton fibers. Two discrete patterns of transcript accumulation were observed. Whereas transcripts of alpha-tubulin genes GhTua2/3 and GhTua4 increased in abundance from 10 to 20 d post anthesis (DPA), GhTua1 and GhTua5 transcripts were abundant only through to 14 DPA, and dropped significantly at 16 DPA with the onset of secondary wall synthesis. This is the first report, to our knowledge, of gene-specific changes in tubulin transcript levels during the development of a terminally differentiated plant cell. The decrease in abundance of GhTua1 and GhTua5 transcripts was correlated with pronounced changes in cell wall structure, suggesting that alpha-tubulin isoforms may be functionally distinct in elongating fiber cells. Although total alpha-tubulin transcript levels were much higher in fiber than several other tissues, including the hypocotyl and pollen, none of the alpha-tubulins was specific to fiber cells.

Isotype-specific changes in the amount of beta-tubulin RNA in synchronized tobacco BY2 cells

The 3'-ends of the beta-tubulin cDNA were amplified from tobacco BY2 polyA+ RNA. According to the differences in the predicted amino acid sequence at the extreme C-terminal, they were grouped into three different isotypes, NTB1 in which "EEGDYYEEDEEDLNEA", NTB2 in which "EEEYYEDEEEA QED" and NTB3 in which "DECEYEEEEEYDHEGN" follows the conservative "YQQYQDATAD" sequence. Using unique 3'-untranslated regions as probes, changes in the RNA levels of each beta-tubulin isotype were determined by dot-blot hybridization. The levels exhibited characteristic rhythms in the cell cycle. NTB1 RNA was highest in S phase in comparison to NTB2 RNA level which was highest in late G2. On the other hand, NTB3 RNA level was highest in early G2.

Functional interactions among cytoskeleton, membranes, and cell wall in the pollen tube of flowering plants

The pollen tube is a cellular system that plays a fundamental role during the process of fertilization in higher plants. Because it is so important, the pollen tube has been subjected to intensive studies with the aim of understanding its biology. The pollen tube represents a fascinating model for studying interactions between the internal cytoskeletal machinery, the membrane system, and the cell wall. These compartments, often studied as independent units, show several molecular interactions and can influence the structure and organization of each other. The way the cell wall is constructed, the dynamics of the endomembrane system, and functions of the cytoskeleton suggest that these compartments are a molecular "continuum," which represents a link between the extracellular environment and the pollen tube cytoplasm. Several experimental approaches have been used to understand how these interactions may translate the pollen-pistil interactions into differential processes of pollen tube growth.

A specific alpha-tubulin is associated with the initiation of parthenogenesis in 'Salmon' wheat lines

The 'Salmon system' consists of isogenic but alloplasmic wheat lines with either sexual or autonomous embryo development. Using two-dimensional gel electrophoresis these lines have been screened for proteins potentially involved in the initiation of parthenogenesis. A temporally altered expression of the polypeptide 'P 115.1' in the sexual and parthenogenetic 'Salmon' lines seems to be related with the autonomous embryo formation. Around anthesis when most of the egg cells begin the parthenogenetic development, the polypeptide 'P 115.1' was present in ovaries of the parthenogenetic lines but not in ovaries of the sexual line. Moreover, this polypeptide is only expressed in the ovaries of amphidiploid parthenogenetic plants containing differentiated embryo sacs. It is absent from ovaries of the analogous polyhaploid plants, which lack any embryo sac structure within their ovules. Furthermore, the polypeptide was neither detectable in meristematic tissue of root tips nor in leaves. N-terminal amino acid sequencing identified 'P 115.1' as an alpha-tubulin. Thus, 'P 115.1' apparently represents an embryo sac-specific isoform of alpha-tubulin involved in the initiation of embryo development.

Endosperm-specific demethylation and activation of specific alleles of alpha-tubulin genes of Zea mays L

We have investigated the methylation status of the alpha-tubulin genes, and the degree of accumulation of their mRNAs in endosperm, embryo and seedling tissues of Zea mays L. We have found that many of the alpha-tubulin genes are differentially demethylated in the endosperm relative to the embryo and seedling. However, only for tub alpha 2 and tub alpha 4 could a correlation between DNA demethylation and increased RNA accumulation be detected. By analyzing the inbred lines W64A and A69Y and their reciprocal crosses, we have also identified in the endosperm two alpha-tubulin genes, tub alpha 3 and tub alpha 4, that are differentially demethylated if transmitted by the maternal germline, but that remain hypermethylated when transmitted by the paternal germline.

The Pisum sativum TubA1 gene, a member of a small family of alpha-tubulin sequences

alpha- and beta-tubulin proteins are subunits of microtubules, which as primary elements of the plant cytoskeleton play major roles in plant cell division and cell morphogenesis. Several higher-plant alpha- and beta-tubulin gene families have been reported to have at least six to nine members each. Using genomic Southern hybridizations and polymerase chain reaction (PCR) experiments, we have found that the Pisum sativum (garden pea) genome has only four copies of alpha-tubulin sequences and a similar number of beta-tubulin sequences. We have characterized the pea alpha-tubulin gene TubA1. Its nucleotide sequence predicts a 452 amino acid product which is 89-98% identical to those predicted for other plant alpha-tubulins. By S1 nuclease analysis we have located the transcript start site at 102 bases upstream of the ATG. We have also shown that the TubA1 gene is expressed by northern hybridization with a gene-specific probe.

Isolation and characterization of two beta-tubulin cDNA clones from rice

Two cDNA clones encoding two different beta-tubulins, RTUB-1 and RTUB-2, were isolated from a rice cDNA library and their nucleotide sequences were analyzed. The deduced amino acid sequences showed amino acid sequence identity between 92% and 97% with other plant beta-tubulins. Southern blot analysis using gene-specific and coding-region probes suggested that beta-tubulins in rice are encoded by multigene families. The two cDNA clones represent two subfamilies of rice tubulins, RTUB-1 and RTUB-2, consisting of 3 to 4 genes and a single gene, respectively. The transcript levels of RTUB-1 and RTUB-2 genes were higher in actively elongating tissues such as etiolated shoot tissues and light-grown root tissues of four-day old seedlings.

Tubulin genes in the algal protist Euglena gracilis

Alpha- and beta-tubulin cDNA were selected from a Euglena lambda gt11 expression library, recloned and either sequenced (alpha-tubulin cDNA) or hybridized to Euglena RNA and DNA (alpha- and beta-tubulin cDNA). RNA for hybridization was extracted at 30 minute intervals after flagellar amputation and quantitated for cDNA binding. Unlike previous reports on most other flagellates, no net increase in either alpha- or beta-tubulin RNA could be detected during regeneration--suggesting steady state or constitutive tubulin RNA synthesis. Incubation of the cDNA with genomic DNA after restriction digestion produced patterns of hybridization consistent with the presence of one to two kinds each of the alpha- and beta-tubulin genes. The deduced amino acid sequence of the alpha-tubulin cDNA was more than 90% identical to the alpha-tubulins of Trypanosoma, Chlamydomonas, Naegleria, Tetrahymena and higher plants. The carboxy terminus of the alpha-tubulin cDNA and the previously sequenced beta-tubulin of Euglena showed greatest identity to the carboxy terminus of the tubulins from Trypanosoma brucei. The sequence data for alpha- and beta-tubulins of Euglena provides direct evidence for the similarity of two gene products from euglenas and trypanosomes and adds support to earlier suggestions that these organisms are phylogenetically related.

A beta-tubulin gene of Naegleria encodes a carboxy-terminal tyrosine. Aromatic amino acids are conserved at carboxy termini

A gene that directs the programmed synthesis of flagellar beta-tubulin during the rapid differentiation of Naegleria gruberi from amoebae to flagellates has been cloned and sequenced. The intronless gene is one of 8 to 10 similar but non-identical genes that are dispersed in the genome. beta-Tubulin mRNA homologous to this gene family is expressed transiently during differentiation, and has not been detected in amoebae. The encoded beta-tubulin is strongly conserved, with features that closely resemble the beta-tubulins of diverse organisms, especially organisms that, like Naegleria, use tubulin to assemble flagellar axonemes. In most sequenced alpha-tubulins, the encoded carboxy-terminal amino acid is tyrosine, which undergoes post-translational removal and readdition, conserved processes of unknown function. In N. gruberi, unusually, the terminus of alpha-tubulin is encoded as glutamine while that of beta-tubulin is tyrosine. The presence of these divergent termini on subunits of a conserved tubulin provoked us to re-examine aromatic amino acids at the termini of alpha- and beta-tubulins. Although evolution has tinkered extensively with the carboxy-terminal domains of tubulin subunits, we find an unexpected conservation. In every organism or cell type for which both tubulin subunits have been sequenced, except the ciliate Stylonychia lemnae, at least one tubulin subunit of some or all tubulin heterodimers terminates in an aromatic amino acid, either tyrosine or phenylalanine. This remarkable conservation of carboxy-terminal aromatic amino acids suggests that these residues serve some crucial function.

Characterization of four new beta-tubulin genes and their expression during male flower development in maize (Zea mays L.)

Four different beta-tubulin coding sequences were isolated from a cDNA library prepared from RNA from maize seedling shoots. The four genes (designated tub4, tub6, tub7 and tub8) represented by these cDNA clones together with the tub1 and tub2 genes reported previously encode six beta-tubulin isotypes with 90-97.5% amino acid sequence identity. Results from phylogenetic analysis of 17 beta-tubulin genes from monocot and dicot plant species indicated that multiple extant lines of beta-tubulin genes diverged from a single precursor after the appearance of the two major subfamilies of alpha-tubulin genes described previously. Hybridization probes from the 3' non-coding regions of six beta-tubulin clones were used to quantify the levels of corresponding tubulin transcripts in different maize tissues including developing anthers and pollen. The results from these dot blot hybridization experiments showed that all of the beta-tubulin genes were expressed in most tissues examined, although each gene showed a unique pattern of differential transcript accumulation. The tub1 gene showed a high level of transcript accumulation in meristematic tissues and almost no accumulation in the late stages of anther development and in pollen. In contrast, the level of tub4 transcripts was very low during early stages of male flower development but increased markedly (more than 100 times) during the development of anthers and in pollen. Results from RNAse protection assays showed that this increased hybridization signal resulted from expression of transcripts from one or two genes closely related to tub4. The tub4-related transcripts were not present in shoot tissue. Transcripts from the tub2 gene accumulated to very low levels in all tissues examined, but reached the highest levels in young anthers containing microspore mother cells. RNAse protection assays were used to measure the absolute levels of alpha- and beta-tubulin transcripts in seedling shoot and in pollen. The alpha-tubulin gene subfamily I genes (tua1, tua2, tua4) contributed the great majority of alpha-tubulin transcripts in both shoot and pollen. Transcripts from the beta-tubulin genes tub4, tub6, tub7, and tub8 were predominant in shoot, but were much less significant than the tub4-related transcripts in pollen.

Development of an homologous transformation system for Acremonium chrysogenum based on the beta-tubulin gene

The beta-tubulin gene was isolated from the filamentous fungus Acremonium chrysogenum using a heterologous gene probe to screen an A. chrysogenum lambda library. Sequencing of the A. chrysogenum gene revealed a mosaic gene which contains five exons and four intervening sequences. The exons encode for a polypeptide of 447 amino-acid residues which showed a high degree of similarity when compared with amino-acid sequences from beta-tubulins of other eukaryotes. The introns are characterized by typical consensus sequences found in intervening sequences from other filamentous fungi. In-vitro mutagenesis of codon 167 of the beta-tubulin gene resulted in the substitution of a phenylalanine by a tyrosine in the corresponding polypeptide sequence. The mutated gene was used successfully in the transformation and co-transformation of A. chrysogenum to benomyl resistance. The molecular analysis of transformants provided evidence that they contain the mutated beta-tubulin gene in addition to the wild-type gene, as was proved by Southern-hybridization analysis and direct sequencing of PCR amplification products.

Antiparallel expression of the sense and antisense transcripts of maize alpha-tubulin genes

In all eukaryotes alpha- and beta-tubulins are encoded by small families of closely related genes and are highly conserved. In Zea mays, at least six different alpha-tubulin coding sequences are known. We describe the isolation from scutellar nodes of the maize inbred line W22 of a clone (CTM5) coding for an alpha-tubulin. On the basis of the 3' end nucleotide sequence, this clone can be assigned to the already reported tua4 gene. Northern analysis demonstrates that CTM5 encodes a 1.5 kb transcript, which is expressed in different tissues of the seed and of the seedling. In order to define the spatial and temporal expression of alpha-tubulin genes, in situ hybridization experiments were performed on these tissues. Unexpectedly, a specific signal was detected with both antisense and sense RNA strands. Temporal and spatial distribution of the two RNAs, however, shows that high levels of the two transcripts are always discordant. In tissues where sense transcripts are highly abundant (embryos at various developmental stages, root tips, pollen grains), the antisense transcripts are expressed in relatively small amounts, while in pericarp, coleoptile, leaves, and scutellar node, where antisense transcripts accumulate, the sense transcript only reaches a very low level. Northern analysis using single-stranded DNA probes confirmed the presence of an antisense transcript of 1.5 kb, prompting speculation about the role of this transcript in the regulation of the expression of alpha-tubulin genes.

A mutation in the alpha 1-tubulin gene of Chlamydomonas reinhardtii confers resistance to anti-microtubule herbicides

A mutation in the alpha 1-tubulin gene of Chlamydomonas reinhardtii was isolated by using the amiprophos-methyl-resistant mutation apm1-18 as a background to select new mutants that showed increased resistance to the drug. The upA12 mutation caused twofold resistance to amiprophos-methyl and oryzalin, and twofold hypersensitivity to the microtubule-stabilizing drug taxol, suggesting that the mutation enhanced microtubule stability. The resistance mutation was semi-dominant and mapped to the same interval on linkage group III as the alpha 1-tubulin gene. Two-dimensional gel immunoblots of proteins in the mutant cells revealed two electrophoretically altered alpha-tubulin isoforms, one of which was acetylated and incorporated into microtubules in the axoneme. The mutant isoforms co-segregated with the drug-resistance phenotypes when mutant upA12 was backcrossed to wild-type cells. Two-dimensional gel analysis of in vitro translation products showed that the non-acetylated variant alpha-tubulin was a primary gene product. DNA sequence analysis of the alpha 1-tubulin genes from mutant and wild-type cells revealed a single missense mutation, which predicted a change in codon 24 from tyrosine in wild type to histidine in mutant upA12. This alteration in the predicted amino acid sequence corroborated the approximately +1 basic charge shift observed for the variant alpha-tubulins. The mutant allele of the alpha 1-tubulin gene was designated tua1-1.

Identification and preliminary characterization of a 65 kDa higher-plant microtubule-associated protein

Microtubules in plant cells, as in animal cells, are dynamic structures. However, our lack of knowledge about the constituents of microtubules in plant cells has prevented us from understanding the mechanisms that control microtubule dynamics. To characterize some of these constituents, a cytoplasmic extract was prepared from evacuolated protoplasts (miniprotoplasts) of tobacco BY-2 cells, and microtubules were assembled in the presence of taxol and disassembled by cold treatment in the presence of Ca2+ and a high concentration of NaCl. SDS-PAGE analysis of triple-cycled microtubule protein revealed the presence of 120 kDa, 110 kDa and a group of 60–65 kDa polypeptides in addition to tubulin. Since these polypeptides had copolymerized with tubulin, through the three cycles of assembly and disassembly, and they bundle microtubules, we tentatively identified the three polypeptides as microtubule-associated proteins (MAPs). To characterize these factors further, triple-cycled microtubule protein was fractionated by Mono-Q anion-exchange chromatography and the microtubule-bundling activity of each fraction was examined. Fractions having microtubule-bundling activity contained only the 65 kDa MAP, an indication that the 65 kDa MAP is responsible for the bundling of microtubules. Purified 65 kDa MAP formed cross-bridge structures between adjacent microtubules in vitro. Polyclonal antibodies were raised in mice against the 65 kDa MAP. Immunofluorescence microscopy revealed that the 65 kDa MAP colocalized with microtubules in BY-2 cells throughout the cell cycle. Western blotting analysis of extracts from several species of plants suggested that the 65 kDa MAP and/or related peptides are widely distributed in the plant kingdom.

Molecular cloning and sequencing analysis of a beta-tubulin gene from Lupinus albus

Genomic lambda-Dash library constructed from Lupinus albus nuclear DNA was screened using a fragment of the beta-tubulin cDNA (beta 8-31) clone of Chlamydomonas reinhardtii as probe. One of the positive recombinant phages was isolated, subcloned and analysed by sequencing. We present here nucleotide and derived amino acid sequences of the beta-tubulin gene, designated as L beta 1 and identified by similarity with other beta-tubulins. The L beta 1-encoded protein reveals a very high degree of similarity with other plant tubulins and contains consensus sequences for binding guanine base, phosphate and Mg2+. Northern analysis of total RNA isolated from roots, leaves, flowers and pools revealed that Lupinus albus beta-tubulin genes are constitutively expressed in all studied plant tissues.

Mapping of beta-tubulin genomic sequences in hexaploid oat (Arena sativa L.)

The allohexaploid nature of Avena sativa L. (2n=6x=42) and the availability of aneuploid lines was exploited in designing a strategy for mapping beta-tubulin sequences in the oat genome. Evidence for a minimum of eight beta-tubulin genes was obtained by Southern-blot analysis. Three betatubulin sequences were localized to chromosomes using DNA from monosomic and nullisomic lines in the variety Sun II. One sequence was localized to the chromosome missing in nullisome I. Two other sequences were mapped to satellite chromosome 2, the chromosome that is missing in nullisome VIII and to which one ribosomal RNA gene cluster had previously been mapped. Restriction fragments carrying these two beta-tubulin genomic sequences and the cluster of ribosomal RNA sequences were missing in DNA from nullisomics VIII, IX and X, suggesting that all three nullisome classes are deficient for an identical chromosomal segment that includes these three loci. This study demonstrates how molecular analyses can be used to characterize aneuploid stocks and to better define their genetic constitution.

Higher plant microtubule-associated proteins: in vitro functional assays

Microtubule-associated proteins (MAPs) can account for the assembly and stabilization of microtubules at low tubulin concentration, for their ability to interact with other microtubules and/or cytoskeletal polymers or organelles and also for regulating microtubule anchoring and bundling properties. The data concerning higher plant MAPs remain limited so far to a few examples. Motor MAPs such as dynein or kinesin remain poorly documented in plants and are not to be discussed here. In this manuscript, the attention is focused on structural MAPs which co-assemble with tubulin during microtubule assembly. Using taxol, we developed an assay where higher plant microtubules were induced to self-assemble in a cytosolic extract of maize cultured cells and could be used as a native matrix for the isolation of putative higher plant MAPs. Seven polypeptides with molecular masses ranging between 60-125 kDa were found in this MAP-enriched fraction. These putative plant MAPs were shown to co-assemble with pig brain tubulin through two cycles of temperature-dependent assembly-disassembly. They were able to initiate and promote MAP-free tubulin assembly under conditions of non-efficient self-assembly and induced bundling of both plant and neural microtubules. One of these polypeptides (83 kDa) was found to be immunologically related to neural tau, suggesting the presence of common epitopes between neural and plant MAPs. Such epitopes may be present at the microtubule-binding domains, as the higher plant MAPs co-assemble with brain tubulin. Plant microtubules exhibit an important in situ bundling activity, as in cortical or pre-prophase band arrays, or during the drastic reorganization of the cytoskeleton during mitosis induction.(ABSTRACT TRUNCATED AT 250 WORDS)

High level expression of nonacetylatable alpha-tubulin in Chlamydomonas reinhardtii

Following the discovery of acetylated alpha-tubulin in the flagella of Chlamydomonas, many studies have documented the presence of acetylated alpha-tubulin in a variety of evolutionarily divergent organisms. While this posttranslational modification may define an isoform with a unique function, the primary effect of alpha-tubulin acetylation remains unknown. To study the function of alpha-tubulin acetylation, we have transformed Chlamydomonas, an organism in which almost all of the flagellar tubulin and a subset of the cytoplasmic microtubules are acetylated, with an alpha 1-tubulin gene whose product cannot be acetylated. Specifically, the codon for lysine 40, the lysine that is acetylated, has been replaced with the codon of nonacetylatable amino acids. To distinguish mutagenized alpha-tubulin from that produced by the two endogenous alpha-tubulin genes, mutant alpha-tubulin was tagged with an epitope from influenza virus hemagglutinin. Utilizing the constitutive Chlamydomonas rubisco small subunit S2 promoter, we have obtained in selected clones high levels of nonacetylatable alpha-tubulin expression approximating 50-70% of the total flagellar alpha-tubulin. Immunofluorescence and immunoblot analysis of transformed cells indicated that nonacetylatable alpha-tubulin could assemble, along with endogenous alpha-tubulin, into both cytoplasmic and flagellar microtubules. However, no gross phenotypic effects were observed, suggesting that the effect of alpha-tubulin acetylation is subtle.

Tubulin gene expression in maize (Zea mays L.). Change in isotype expression along the developmental axis of seedling root

Two-dimensional gel/western blot analysis was used to characterize alpha- and beta-tubulin isotype expression along the developmental axis of the maize (Zea mays) seedling primary root. We identified four distinct alpha-tubulin isotypes and a minimum of six beta-tubulin isotypes. This analysis showed differences between the alpha- and beta-tubulin isotypes expressed in rapidly dividing tissue at the root tip and differentiated root tissues proximal to the tip. The alpha 1 and alpha 4 isotypes predominated in samples from immature rapidly dividing tissues such as root tips, whereas in mature tissues such as differentiated root and pollen, alpha 2, alpha 3 and alpha 4 isotypes predominated. The beta 1 and beta 2 isotypes were more abundant in protein samples from root cortex than in samples from the root tip or vascular cylinder. In contrast, the beta 4 and beta 5 isotypes appeared to be more abundant in root tip and vascular cylinder samples than in root cortex samples. Hybridization probes from the 3' non-coding region of six alpha-tubulin cDNA clones were used to quantify the levels of corresponding tubulin transcripts in selected tissues, from embryonic to mature and from largely undifferentiated to highly differentiated. The results from these hybridization experiments showed that all of the alpha-tubulin genes were expressed in all tissues examined, although each gene showed a unique pattern of differential transcript accumulation. A transcript produced from cDNA clone representing the tua5 alpha-tubulin gene was translated in vitro and produced an alpha-tubulin that comigrated with the alpha 2 isotype.

Alpha-tubulin gene family of maize (Zea mays L.). Evidence for two ancient alpha-tubulin genes in plants

Among 81 alpha-tubulin cDNA clones prepared from RNA from maize seedling shoot, endosperm and pollen, we identified six different alpha-tubulin coding sequences. The DNA sequence analysis of coding and non-coding regions from the clones showed that they can be classified into three different alpha-tubulin gene subfamilies. Genes within each subfamily encode proteins that are 99 to 100% identical in amino acid sequence. Deduced amino acid sequence identity between genes in different subfamilies ranges from 89 to 93%. The results of hybridizations of genomic DNAs to alpha-tubulin coding region probes and to 3' non-coding region probes constructed from six different alpha-tubulin cDNA clones indicated that the maize alpha-tubulin gene family contains at least eight members. Comparison of deduced alpha-tubulin amino acid sequences from maize and the dicot species Arabidopsis thaliana showed that alpha-tubulin isotypes encoded by genes in maize subfamilies I and II are more similar to specific Arabidopsis gene products (96 to 97% amino acid identity) than to isotypes encoded by genes in the other maize subfamilies. Phylogenetic analyses revealed that genes in these two subfamilies were derived from two ancient alpha-tubulin genes that predate the divergence of monocots and dicots. These same analyses revealed that the gene in maize subfamily III is more closely related to sequences from subfamily I genes than to those from subfamily II genes. However, the subfamily III gene has no close counterpart in Arabidopsis. We found evidence of a transposable element-like insertion in the subfamily III gene in some maize lines.

Amino acid alterations in the benA (beta-tubulin) gene of Aspergillus nidulans that confer benomyl resistance

We report the cloning and sequencing of 18 mutant alleles of the benA, beta-tubulin gene of Aspergillus nidulans that confer resistance to the benzimidazole antifungal, antimicrotubule compounds benomyl, carbendazim, nocodazole, and thiabendazole. In 12 cases, amino acid 6 was changed from histidine to tyrosine or leucine. In four cases, amino acid 198 was changed from glutamic acid to aspartic acid, glutamine, or lysine. In two cases, amino acid 200 was altered from phenylalanine to tyrosine. These data, along with previous data indicating that amino acid 165 is involved in the binding of the R2 group of these compounds [Jung and Oakley, 1990: Cell Motil. Cytoskeleton 17:87-94], suggest that regions of beta-tubulin containing amino acids 6, 165, and 198-200 interact to form the binding site of benzimidazole antimicrotubule agents. These results also suggest that the presence of phenylalanine at amino acid 200 contributes to the great sensitivity of many fungi to benzimidazole antimicrotubule agents.

Different patterns of expression of beta-tubulin genes in Tetrahymena pyriformis during reciliation

The ciliate Tetrahymena pyriformis contains one alpha-tubulin (alpha TT) and two beta-tubulin (beta TT1 and beta TT2) genes. The specific expression of these genes was investigated by Northern blot hybridization using oligonucleotide probes complementary to beta TT1 and beta TT2 genes and the coding region of the alpha-tubulin gene. The three genes are expressed producing 1.8-kb mRNAs but the level of beta TT1 mRNA is much higher than that of beta TT2 mRNA. During cilia regeneration, we found that the expression patterns of the alpha TT and beta TT1 genes are similar whereas that of the beta TT2 gene is different. The alpha TT and beta TT1 transcripts reached higher values between 60-120 min after the onset of reciliation than in exponentially growing cells, while beta TT2 transcripts were maintained at low levels during the whole period. The differences in the amounts of steady-state populations of the both beta-tubulin mRNAs do not correspond to the copy number per haploid genome. These differences could result from the fact that the promoter region of beta TT2 may contain highly structured sequences which would affect the binding of the respective trans-acting factor(s). The apparent transcription rate revealed a significant increase at 15 min of reciliation which could be responsible for the high levels of alpha TT and beta TT1 transcripts in the cytoplasm between 60-120 min of reciliation. This coordinated response to cilia regeneration of the alpha TT and beta TT1 tubulin genes is also a relevant aspect of our findings. Several conserved motifs found in their promoter regions led us to think that some of them may function as cis-elements in the specific binding of nuclear protein factor(s).

Limited expression of a diverged beta-tubulin gene during soybean (Glycine max [L.] Merr.) development

We examined the developmental expression of a diverged soybean beta-tubulin gene (designated sb-1), which had been cloned and sequenced previously. A probe specific for the sb-1 gene was constructed from the 3' transcribed untranslated sequence. As a control, a more general probe for beta-tubulin genes and their transcripts was constructed from a highly conserved region of the third exon of another soybean beta-tubulin gene, sb-2. Poly(A)+ RNA, extracted from various soybean tissues and organs, was probed alternatively with the sb-1 gene-specific probe and with the generic beta-tubulin probe. Levels of beta-tubulin transcripts recognized by the generic probe differed by a factor of approximately 3 in the different tissues and organs and varied with the state of organ development. Highest levels were found in young, unexpanded leaves and they decreased as leaf maturation occurred. In contrast, transcripts of sb-1 were nearly undetectable in young leaves, and they increased as leaf maturation occurred. Levels of sb-1 transcript were low in all organs of the light-grown plant examined, except the hypocotyl, where they were approximately 10-fold higher. However, the highest levels of sb-1 transcripts were observed in elongating hypocotyls of etiolated seedlings. Exposure of six-day-old etiolated seedlings to light for 12 hours halted further hypocotyl elongation and brought about a dramatic, nearly 100-fold, decrease in the steady-state level of sb-1 transcripts.

The biochemistry of compounds with anti-microtubule activity in plant cells

The experimental use of anti-microtubule compounds has revealed essential functions of microtubules in plant cytoskeletal arrays, including the pre-prophase band, the mitotic and meiotic spindles, the phragmoplast, and the cortical array. The most commonly used plant microtubule depolymerization compounds are colchicine, and several synthetic herbicides belonging to three different chemical classes, the dinitroanilines, phosphoric amides, and N-phenyl carbamates. Taxol, a secondary plant product, is the only drug found to promote the polymerization of plant microtubules. This paper summarizes our current understanding of the biochemical interactions of colchicine, anti-microtubule herbicides, and taxol with plant tubulin and microtubules.

Multiple polyadenylation sites are active in the alpha 1-tubulin gene from Zea mays

Two cDNA clones (MR2 and MR29) encoding the same alpha-tubulin isotype (alpha 1) have been identified and characterized from maize roots. The sequence of these two cDNA clones is identical to a previously described cDNA clone (MR19) except in the location of their polyadenylation site. The sequence of the cDNA and its homologous genomic clone (MG19/14) shows two putative polyadenylation signals which could direct the variable 3' processing of the observed transcripts. Endonuclease S1 protection analysis in this 3' flanking region confirms the presence in the alpha 1-tubulin gene from Zea mays of these two main functional polyadenylation sites and possibly other related ones. The relative accumulation of RNAs bearing the two main polyadenylation sites has been tested by using a RNA-slot analysis of several tissues of the plant. It appears that a higher proportion of shorter mRNA species is found in actively dividing tissues.

The Tub alpha 3 gene from Zea mays: structure and expression in dividing plant tissues

A gene (Tub alpha 3) coding for an alpha-Tub, expressed in dividing tissues, has been cloned from Zea mays. The deduced amino acid (aa) sequence, 450 aa long, is very similar to the other plant alpha-Tub (85-89% homology) so far reported, and in particular to the other two aa sequences (alpha 1-Tub and alpha 2-Tub) already published from the same species (93% homology). The genomic structure is also very similar, having three introns located at the same positions as in the Tub alpha 1 and Tub alpha 2 genes, one of them placed at the same position in the homologous genes from Arabidopsis thaliana. Nevertheless, the noncoding sequences are very different from the two other maize genomic sequences. In particular, no homology has been found either in the 5' upstream or in the 3'-untranslated sequences. Using specific 3' probes, it has been possible to detect the mRNA coded by this gene in many of the plant organs measured, but its highest abundance is observed in the organs rich in dividing cells, a pattern correlated with that of the histone H4-encoding gene. A cDNA clone has been identified in maize coleoptiles and sequenced, confirming the expression of the Tub alpha 3 in this organ. No preferential accumulation in any organ of the plant was found, in contrast with what was observed in the Tub alpha 1 and Tub alpha 2 genes already described. The Tub alpha gene family seems to consist in maize by at least two groups of homologous sequences, each one including a maximum of two or three coding units.

Analysis of beta-tubulin sequences reveals highly conserved, coordinated amino acid substitutions. Evidence that these 'hot spots' are directly involved in the conformational change required for dynamic instability

Vertebrate beta-tubulins have been classified into six classes on the basis of their C-terminal sequences [(1987) J. Cell Biol. 105, 1707-1720]. In particular, the sequences starting at residue 430 differ between isotypes of the same animal but are conserved between species. We extend this analysis and show that there are three 'hot spots', at residues 35, 55-57 and 124 which exhibit intra-species heterogeneity but inter-species conservation. There is a remarkable correlation between the identity of these residues and the C-terminal sequences, and suggests that the vertebrate beta-tubulins fall into three broad types. This correlation extends to those non-vertebrate organisms which have the Type 1 C-terminal sequence. We propose that these three 'hot spots' and the C-terminal peptide interact in the tertiary structure. We have also noted that the C-terminal peptide almost always contains a single phenylalanine or tyrosine residue, and that there is a strong correlation between this residue and the amino acids at positions 217/218, in both the vertebrate and non-vertebrate sequences. We propose that the C-terminal aromatic amino acid interacts with residues 217/218 in the tertiary structure. Analysis of conditions which stabilise microtubules and/or lower the steady state critical concentration strongly suggests that these two sets of coordinated amino acid substitutions are directly involved in effecting the conformational change associated with GTP hydrolysis which results in dynamic instability. We propose that there is an interaction between the highly acidic sequence between residue 430 and the aromatic amino acid (termed peptide A) and conserved basic amino acids located close to the 'hot spots'. We suggest that this interaction is altered in response to the assembly-dependent GTP hydrolysis, with the consequential increase in the subunit dissociation rate constant.

A tandem of alpha-tubulin genes preferentially expressed in radicular tissues from Zea mays

The identification of a cDNA (MR19) corresponding to a maize alpha-tubulin and homologous genomic clones (MG19/6 and MG19/14) is described. The cDNA has been isolated by differential screening of a cDNA maize root library. We have found two alpha-tubulin genes in a tandem arrangement in the genomic clones, separated by approximately 1.5 kbp. One of the genes (gene I) contains an identical nucleotide sequence which corresponds to the cDNA clone. The two deduced proteins from DNA sequences are very similar (only two conservative replacements in 451 amino acids) and they share a high homology as compared with the published alpha-tubulin sequences from other systems and in particular with the Arabidopsis thaliana and Chlamydomonas reinhardtii sequences reported. The structure of both genes is also very similar; it includes two introns, of 1.7 kbp and 0.8 kbp respectively, in each gene and only one intron placed at a homologous position in relation to Arabidopsis thaliana genes. By using specific 3' probes it appears that both genes are preferentially expressed in the radicular system of the plant. The alpha-tubulin gene family of Zea mays seems to be represented by at least 3 or 4 members.

Identification of an amino acid substitution in the benA, beta-tubulin gene of Aspergillus nidulans that confers thiabendazole resistance and benomyl supersensitivity

We are using molecular genetic techniques to identify sites of interaction of beta-tubulin with benzimidizole anti-microtubule agents. We have developed a marker-rescue technique for cloning mutant alleles of the benA, beta-tubulin gene of Aspergillus nidulans and have used the technique to clone two mutant benA alleles, benA16 and benA19. These are the only A. nidulans alleles known to confer resistance to the benzimidazole antimicrotubule agent thiabendazole and supersensitivity to other benzimidazole antimicrotubule agents including benomyl and its active breakdown product, carbendazim. benA16 has been shown, moreover, to reduce thiabendazole binding to beta-tubulin. We have sequenced the two mutant alleles and have found that they carry different nucleotide changes that cause the same single amino acid substitution, valine for alanine at amino acid 165. Since thiabendazole and carbendazim differ at only one side chain, the R2 group, we conclude that the region around amino acid 165 is involved in the binding of the R2 group of benzimidazole antimicrotubule agents to beta-tubulin.