Two alpha-tubulin genes of Aspergillus nidulans encode divergent proteins

Aspergillus SUMOylation mutants exhibit chromosome segregation defects including chromatin bridges

Functions of protein SUMOylation remain incompletely understood in different cell types. Via forward genetics, here we identified ubaBQ247*, a loss-of-function mutation in a SUMO activation enzyme UbaB in the filamentous fungus Aspergillus nidulans. The ubaBQ247*, ΔubaB, and ΔsumO mutants all produce abnormal chromatin bridges, indicating the importance of SUMOylation in the completion of chromosome segregation. The bridges are enclosed by nuclear membrane containing peripheral nuclear pore complex proteins that normally get dispersed during mitosis, and the bridges are also surrounded by cytoplasmic microtubules typical of interphase cells. Time-lapse sequences further indicate that most bridges persist through interphase prior to the next mitosis, and anaphase chromosome segregation can produce new bridges that persist into the next interphase. When the first mitosis happens at a higher temperature of 42°C, SUMOylation deficiency produces not only chromatin bridges but also many abnormally shaped single nuclei that fail to divide. UbaB-GFP localizes to interphase nuclei just like the previously studied SumO-GFP, but the nuclear signals disappear during mitosis when the nuclear pores are partially open, and the signals reappear after mitosis. The nuclear localization is consistent with many SUMO targets being nuclear proteins. Finally, although the budding yeast SUMOylation machinery interacts with LIS1, a protein critical for dynein activation, loss of SUMOylation does not cause any obvious defect in dynein-mediated transport of nuclei and early endosomes, indicating that SUMOylation is unnecessary for dynein activation in A. nidulans.

Microtubules in Microorganisms: How Tubulin Isotypes Contribute to Diverse Cytoskeletal Functions

The cellular functions of the microtubule (MT) cytoskeleton range from relatively simple to amazingly complex. Assembled from tubulin, a heterodimeric protein with α- and β-tubulin subunits, microtubules are long, hollow cylindrical filaments with inherent polarity. They are intrinsically dynamic polymers that utilize GTP binding by tubulin, and subsequent hydrolysis, to drive spontaneous assembly and disassembly. Early studies indicated that cellular MTs are composed of multiple variants, or isotypes, of α- and β-tubulins, and that these multi-isotype polymers are further diversified by a range of posttranslational modifications (PTMs) to tubulin. These findings support the multi-tubulin hypothesis whereby individual, or combinations of tubulin isotypes possess unique properties needed to support diverse MT structures and/or cellular processes. Beginning 40 years ago researchers have sought to address this hypothesis, and the role of tubulin isotypes, by exploiting experimentally accessible, genetically tractable and functionally conserved model systems. Among these systems, important insights have been gained from eukaryotic microbial models. In this review, we illustrate how using microorganisms yielded among the earliest evidence that tubulin isotypes harbor distinct properties, as well as recent insights as to how they facilitate specific cellular processes. Ongoing and future research in microorganisms will likely continue to reveal basic mechanisms for how tubulin isotypes facilitate MT functions, along with valuable perspectives on how they mediate the range of conserved and diverse processes observed across eukaryotic microbes.

Tubulin isotypes - functional insights from model organisms

The microtubule cytoskeleton is assembled from the α- and β-tubulin subunits of the canonical tubulin heterodimer, which polymerizes into microtubules, and a small number of other family members, such as γ-tubulin, with specialized functions. Overall, microtubule function involves the collective action of multiple α- and β-tubulin isotypes. However, despite 40 years of awareness that most eukaryotes harbor multiple tubulin isotypes, their role in the microtubule cytoskeleton has remained relatively unclear. Various model organisms offer specific advantages for gaining insight into the role of tubulin isotypes. Whereas simple unicellular organisms such as yeast provide experimental tractability that can facilitate deeper access to mechanistic details, more complex organisms, such as the fruit fly, nematode and mouse, can be used to discern potential specialized functions of tissue- and structure-specific isotypes. Here, we review the role of α- and β-tubulin isotypes in microtubule function and in associated tubulinopathies with an emphasis on the advances gained using model organisms. Overall, we argue that studying tubulin isotypes in a range of organisms can reveal the fundamental mechanisms by which they mediate microtubule function. It will also provide valuable perspectives on how these mechanisms underlie the functional and biological diversity of the cytoskeleton.

Disruption of alpha-tubulin releases carbon catabolite repression and enhances enzyme production in Trichoderma reesei even in the presence of glucose

BACKGROUND

Trichoderma reesei is a filamentous fungus that is important as an industrial producer of cellulases and hemicellulases due to its high secretion of these enzymes and outstanding performance in industrial fermenters. However, the reduction of enzyme production caused by carbon catabolite repression (CCR) has long been a problem. Disruption of a typical transcriptional regulator, Cre1, does not sufficiently suppress this reduction in the presence of glucose.

RESULTS

We found that deletion of an α-tubulin (tubB) in T. reesei enhanced both the amount and rate of secretory protein production. Also, the tubulin-disrupted (ΔtubB) strain had high enzyme production and the same enzyme profile even if the strain was cultured in a glucose-containing medium. From transcriptome analysis, the ΔtubB strain exhibited upregulation of both cellulase and hemicellulase genes including some that were not originally induced by cellulose. Moreover, cellobiose transporter genes and the other sugar transporter genes were highly upregulated, and simultaneous uptake of glucose and cellobiose was also observed in the ΔtubB strain. These results suggested that the ΔtubB strain was released from CCR.

CONCLUSION

Trichoderma reesei α-tubulin is involved in the transcription of cellulase and hemicellulase genes, as well as in CCR. This is the first report of overcoming CCR by disrupting α-tubulin gene in T. reesei. The disruption of α-tubulin is a promising approach for creating next-generation enzyme-producing strains of T. reesei.

Conditional promoters to investigate gene function during wheat infection by Zymoseptoria tritici

The fungus Zymoseptoria tritici causes Septoria tritici leaf blotch, which poses a serious threat to temperate-grown wheat. Recently, we described a raft of molecular tools to study the biology of this fungus in vitro. Amongst these are 5 conditional promoters (Pnar1, Pex1A, Picl1, Pgal7, PlaraB), which allow controlled over-expression or repression of target genes in cells grown in liquid culture. However, their use in the host-pathogen interaction in planta was not tested. Here, we investigate the behaviour of these promoters by quantitative live cell imaging of green-fluorescent protein-expressing cells during 6 stages of the plant infection process. We show that Pnar1 and Picl1 are repressed in planta and demonstrate their suitability for studying essential gene expression and function in plant colonisation. The promoters Pgal7 and Pex1A are not fully-repressed in planta, but are induced during pycnidiation. This indicates the presence of inducing galactose or xylose and/or arabinose, released from the plant cell wall by the activity of fungal hydrolases. In contrast, the PlaraB promoter, which normally controls expression of an α-l-arabinofuranosidase B, is strongly induced inside the leaf. This suggests that the fungus is exposed to L-arabinose in the mesophyll apoplast. Taken together, this study establishes 2 repressible promoters (Pnar1 and Picl1) and three inducible promoters (Pgal7, Pex1A, PlaraB) for molecular studies in planta. Moreover, we provide circumstantial evidence for plant cell wall degradation during the biotrophic phase of Z. tritici infection.

α1-Tubulin FaTuA1 plays crucial roles in vegetative growth and conidiation in Fusarium asiaticum

The filamentous ascomycete Fusarium asiaticum contains two homologous genes FaTUA1 and FaTUA2 encoding α-tubulins. In this study, we found that FaTUA2 was dispensable for vegetative growth and sporulation in F. asiaticum. The deletion of FaTUA1 however led to dramatically reduced mycelial growth, twisted hyphae and abnormal nuclei in apical cells of hyphae. The FaTUA1 deletion mutant (ΔFaTuA1-5) also showed a significant decrease in conidiation, and produced abnormal conidia. Pathogenicity assays showed that ΔFaTuA1-5 exhibited decreased virulence on wheat head. Unexpectedly, the deletion of FaTUA1 led to resistance to high temperatures. In addition, ΔFaTuA2 showed increased sensitivity to carbendazim. Furthermore, increased FaTUA2 expression in ΔFaTuA1-5 partially restored the defects of the mutant in mycelial growth, conidial production and virulence, vice versa, increased FaTUA1 expression in the FaTUA2 deletion mutant also partially relieved the defect of the mutant in the delay of conidial germination. Taken together, these results indicate that FaTuA1 plays crucial roles in vegetative growth and development, and the functions of FaTuA1 and FaTuA2 are partially interchangeable in F. asiaticum.

Molecular evolution and functional divergence of tubulin superfamily in the fungal tree of life

Microtubules are essential for various cellular activities and β-tubulins are the target of benzimidazole fungicides. However, the evolution and molecular mechanisms driving functional diversification in fungal tubulins are not clear. In this study, we systematically identified tubulin genes from 59 representative fungi across the fungal kingdom. Phylogenetic analysis showed that α-/β-tubulin genes underwent multiple independent duplications and losses in different fungal lineages and formed distinct paralogous/orthologous clades. The last common ancestor of basidiomycetes and ascomycetes likely possessed two paralogs of α-tubulin (α₁/α₂) and β-tubulin (β₁/β₂) genes but α₂-tubulin genes were lost in basidiomycetes and β₂-tubulin genes were lost in most ascomycetes. Molecular evolutionary analysis indicated that α₁, α₂, and β₂-tubulins have been under strong divergent selection and adaptive positive selection. Many positively selected sites are at or adjacent to important functional sites and likely contribute to functional diversification. We further experimentally confirmed functional divergence of two β-tubulins in Fusarium and identified type II variations in FgTub2 responsible for function shifts. In this study, we also identified δ-/ε-/η-tubulins in Chytridiomycetes. Overall, our results illustrated that different evolutionary mechanisms drive functional diversification of α-/β-tubulin genes in different fungal lineages, and residues under positive selection could provide targets for further experimental study.

Overexpression of the Aspergillus nidulans histone 4 acetyltransferase EsaA increases activation of secondary metabolite production

Regulation of secondary metabolite (SM) gene clusters in Aspergillus nidulans has been shown to occur through cluster-specific transcription factors or through global regulators of chromatin structure such as histone methyltransferases, histone deacetylases, or the putative methyltransferase LaeA. A multicopy suppressor screen for genes capable of returning SM production to the SM deficient ΔlaeA mutant resulted in identification of the essential histone acetyltransferase EsaA, able to complement an esa1 deletion in Saccharomyces cereviseae. Here we report that EsaA plays a novel role in SM cluster activation through histone 4 lysine 12 (H4K12) acetylation in four examined SM gene clusters (sterigmatocystin, penicillin, terrequinone and orsellinic acid), in contrast to no increase in H4K12 acetylation of the housekeeping tubA promoter. This augmented SM cluster acetylation requires LaeA for full effect and correlates with both increased transcript levels and metabolite production relative to wild type. H4K12 levels may thus represent a unique indicator of relative production potential, notably of SMs.

Central roles of small GTPases in the development of cell polarity in yeast and beyond

SUMMARY

The establishment of cell polarity is critical for the development of many organisms and for the function of many cell types. A large number of studies of diverse organisms from yeast to humans indicate that the conserved, small-molecular-weight GTPases function as key signaling proteins involved in cell polarization. The budding yeast Saccharomyces cerevisiae is a particularly attractive model because it displays pronounced cell polarity in response to intracellular and extracellular cues. Cells of S. cerevisiae undergo polarized growth during various phases of their life cycle, such as during vegetative growth, mating between haploid cells of opposite mating types, and filamentous growth upon deprivation of nutrition such as nitrogen. Substantial progress has been made in deciphering the molecular basis of cell polarity in budding yeast. In particular, it becomes increasingly clear how small GTPases regulate polarized cytoskeletal organization, cell wall assembly, and exocytosis at the molecular level and how these GTPases are regulated. In this review, we discuss the key signaling pathways that regulate cell polarization during the mitotic cell cycle and during mating.

Cell polarity in filamentous fungi: shaping the mold

The formation of highly polarized hyphae that grow by apical extension is a defining feature of the filamentous fungi. High-resolution microscopy and mathematical modeling have revealed the importance of the cytoskeleton and the Spitzenkorper (an apical vesicle cluster) in hyphal morphogenesis. However, the underlying molecular mechanisms remain poorly characterized. In this review, the pathways and functions known to be involved in polarized hyphal growth are summarized. A central theme is the notion that the polarized growth of hyphae is more complex than in yeast, though similar sets of core pathways are likely utilized. In addition, a model for the establishment and maintenance of hyphal polarity is presented. Key features of the model include the idea that polarity establishment is a stochastic process that occurs independent of internal landmarks. Moreover, the stabilization of nascent polarity axes may be the critical step that permits the emergence of a new hypha.

Polarity in filamentous fungi: moving beyond the yeast paradigm

Filamentous fungi grow by the polar extension of hyphae. This polar growth requires the specification of sites of germ tube or branch emergence, followed by the recruitment of the morphogenetic machinery to those sites for localized cell wall deposition. Researchers attempting to understand hyphal morphogenesis have relied upon the powerful paradigm of bud emergence in the yeast Saccharomyces cerevisiae. The yeast paradigm has provided a useful framework, however several features of hyphal morphogenesis, such as the ability to maintain multiple axes of polarity and an extremely rapid extension rate, cannot be explained by simple extrapolation from yeast models. We discuss recent polarity research from filamentous fungi focusing on the position of germ tube emergence, the relaying of positional information via RhoGTPase modules, and the recruitment of morphogenetic machinery components including cytoskeleton, polarisome and ARP2/3 complexes, and the vesicle trafficking system.

Tubulins in Aspergillus nidulans

The discovery and characterization of the tubulin superfamily in Aspergillus nidulans is described. Remarkably, the genes that encode alpha-, beta-, and gamma-tubulins were all identified first in A. nidulans. There are two alpha-tubulin genes, tubA and tubB, two beta-tubulin genes, benA and tubC, and one gamma-tubulin gene, mipA. Hyphal tubulin is encoded mainly by the essential genes tubA and benA. TubC is expressed during conidiation and tubB is required for the sexual cycle. Promoter swapping experiments indicate that the alpha-tubulins encoded by tubA and tubB are functionally interchangeable as are the beta-tubulins encoded by benA and tubC. BenA mutations that alter resistance to benzimidazole antimicrotubule agents are clustered and define a putative binding region for these compounds. gamma-Tubulin localizes to the spindle pole body and is essential for mitotic spindle formation. The phenotypes of mipA mutants suggest, moreover, that gamma-tubulin has essential functions in addition to microtubule nucleation.

TINA interacts with the NIMA kinase in Aspergillus nidulans and negatively regulates astral microtubules during metaphase arrest

The tinA gene of Aspergillus nidulans encodes a protein that interacts with the NIMA mitotic protein kinase in a cell cycle-specific manner. Highly similar proteins are encoded in Neurospora crassa and Aspergillus fumigatus. TINA and NIMA preferentially interact in interphase and larger forms of TINA are generated during mitosis. Localization studies indicate that TINA is specifically localized to the spindle pole bodies only during mitosis in a microtubule-dependent manner. Deletion of tinA alone is not lethal but displays synthetic lethality in combination with the anaphase-promoting complex/cyclosome mutation bimE7. At the bimE7 metaphase arrest point, lack of TINA enhanced the nucleation of bundles of cytoplasmic microtubules from the spindle pole bodies. These microtubules interacted to form spindles joined in series via astral microtubules as revealed by live cell imaging. Because TINA is modified and localizes to the spindle pole bodies at mitosis, and lack of TINA causes enhanced production of cytoplasmic microtubules at metaphase arrest, we suggest TINA is involved in negative regulation of the astral microtubule organizing capacity of the spindle pole bodies during metaphase.

Microtubule dynamics during infection-related morphogenesis of Colletotrichum lagenarium

Using a green fluorescent protein (GFP)-tubulin fusion protein, we have investigated the dynamic rearrangement of microtubules during appressorium formation of Colletotrichum lagenarium. Two alpha-tubulin genes of C. lagenarium were isolated, and GFP-alpha-tubulin protein was expressed in this fungus. The strain expressing the fusion protein formed fluorescent filaments that were disrupted by a microtubule-depolymerizing drug, benomyl, demonstrating successful visualization of microtubules. In preincubated conidia, GFP-labeled interphase microtubules, showing random orientation, were observed. At conidial germination, microtubules oriented toward a germination site. At nuclear division, when germ tubes had formed appressoria, mitotic spindles appeared inside conidia followed by disassembly of interphase microtubules. Remarkably, time-lapse views showed that interphase microtubules contact a microtubule-associated center at the cell cortex of conidia that is different from a nuclear spindle pole body (SPB) before their disassembly. Duplicated nuclear SPBs separately moved toward conidium and appressorium accompanied by astral microtubule formation. Benomyl treatment caused movement of both daughter nuclei into 70% of appressoria and affected appressorium morphogenesis. In conidia elongating hyphae without appressoria, microtubules showed polar elongation which is distinct from their random orientation inside appressoria.

Isolation and characterization of alpha-tubulin genes from Septoria tritici and Rhynchosporium secalis, and comparative analysis of fungal alpha-tubulin sequences

The alpha-tubulin genes from Septoria tritici and Rhynchosporium secalis have been cloned and sequenced. The predicted amino acid sequence and intron structure showed strong homology with other known filamentous fungal alpha-tubulins. Comparison of sixteen fungal alpha-tubulin sequences based on amino acid sequence homology and intron structure identified five groups of proteins. Group 1 consists of filamentous fungi, including S. tritici and R. secalis, the dimorphic fungus Histoplasma capsulatum, and Pneumocystis carinii. Group 2 includes two divergent isoforms from Neurospora crassa and Aspergillus nidulans. Group 3 includes the yeast Saccharomyces cerevisiae and the dimorphic fungus Candida albicans. Group 4 contains the single yeast Schizosaccharomyces pombe. Group 5 includes the only Basidiomycete, Schizophyllum commune. This analysis supports the classification of P carinii as a primitive Ascomycete. The presence of an additional glycine residue between the second and third amino acid found only in Group 2 proteins may indicate a functionally distinct fungal isotype. Implications in terms of structure-function relationships for alpha-tubulin molecules are discussed.

Mitosis in filamentous fungi: how we got where we are

This review traces the principal advances in the study of mitosis in filamentous fungi from its beginnings near the end of the 19(th) century to the present day. Meiosis and mitosis had been accurately described and illustrated by the second decade of the present century and were known to closely resemble nuclear divisions in higher eukaryotes. This information was effectively lost in the mid-1950s, and the essential features of mitosis were then rediscovered from about the mid-1960s to the mid-1970s. Interest in the forces that separate chromatids and spindle poles during fungal mitosis followed closely on the heels of detailed descriptions of the mitotic apparatus in vivo and ultrastructurally during this and the following decade. About the same time, fundamental studies of the structure of fungal chromatin and biochemical characterization of fungal tubulin were being carried out. These cytological and biochemical studies set the stage for a surge of renewed interest in fungal mitosis that was issued in by the age of molecular biology. Filamentous fungi have provided model studies of the cytology and genetics of mitosis, including important advances in the study of mitotic forces, microtubule-associated motor proteins, and mitotic regulatory mechanisms.

The pro1(+) gene from Sordaria macrospora encodes a C6 zinc finger transcription factor required for fruiting body development

During sexual morphogenesis, the filamentous ascomycete Sordaria macrospora differentiates into multicellular fruiting bodies called perithecia. Previously it has been shown that this developmental process is under polygenic control. To further understand the molecular mechanisms involved in fruiting body formation, we generated the protoperithecia forming mutant pro1, in which the normal development of protoperithecia into perithecia has been disrupted. We succeeded in isolating a cosmid clone from an indexed cosmid library, which was able to complement the pro1(-) mutation. Deletion analysis, followed by DNA sequencing, subsequently demonstrated that fertility was restored to the pro1 mutant by an open reading frame encoding a 689-amino-acid polypeptide, which we named PRO1. A region from this polypeptide shares significant homology with the DNA-binding domains found in fungal C6 zinc finger transcription factors, such as the GAL4 protein from yeast. However, other typical regions of C6 zinc finger proteins, such as dimerization elements, are absent in PRO1. The involvement of the pro1(+) gene in fruiting body development was further confirmed by trying to complement the mutant phenotype with in vitro mutagenized and truncated versions of the pro1 open reading frame. Southern hybridization experiments also indicated that pro1(+) homologues are present in other sexually propagating filamentous ascomycetes.

Nuclear movement in filamentous fungi

One of the most striking features of eukaryotic cells is the organization of specific functions into organelles such as nuclei, mitochondria, chloroplasts, the endoplasmic reticulum, vacuoles, peroxisomes or the Golgi apparatus. These membrane-surrounded compartments are not synthesized de novo but are bequeathed to daughter cells during cell division. The successful transmittance of organelles to daughter cells requires the growth, division and separation of these compartments and involves a complex machinery consisting of cytoskeletal components, mechanochemical motor proteins and regulatory factors. Organelles such as nuclei, which are present in most cells in a single copy, must be precisely positioned prior to cytokinesis. In many eukaryotic cells the cleavage plane for cell division is defined by the location of the nucleus prior to mitosis. Nuclear positioning is thus absolutely crucial in the unequal cell divisions that occur during development and embryogenesis. Yeast and filamentous fungi are excellent organisms for the molecular analysis of nuclear migration because of their amenability to a broad variety of powerful analytical methods unavailable in higher eukaryotes. Filamentous fungi are especially attractive models because the longitudinally elongated cells grow by apical tip extension and the organelles are often required to migrate long distances. This review describes nuclear migration in filamentous fungi, the approaches used for and the results of its molecular analysis and the projection of the results to other organisms.

Multiple forms of tubulin: different gene products and covalent modifications

Tubulin, the subunit protein of microtubules, is an alpha/beta heterodimer. In many organisms, both alpha and beta exist in numerous isotypic forms encoded by different genes. In addition, both alpha and beta undergo a variety of posttranslational covalent modifications, including acetylation, phosphorylation, detyrosylation, polyglutamylation, and polyglycylation. In this review the distribution and possible functional significance of the various forms of tubulin are discussed. In analyzing the differences among tubulin isotypes encoded by different genes, some appear to have no functional significance, some increase the overall adaptability of the organism to environmental challenges, and some appear to perform specific functions including formation of particular organelles and interactions with specific proteins. Purified isotypes also display different properties in vitro. Although the significance of all the covalent modification of tubulin is not fully understood, some of them may influence the stability of modified microtubules in vivo as well as interactions with certain proteins and may help to determine the functional role of microtubules in the cell. The review also discusses isotypes of gamma-tubulin and puts various forms of tubulin in an evolutionary context.

Isolation and characterization of a gene encoding alpha-tubulin from Candida albicans

A gene encoding the alpha-tubulin of Candida albicans has been cloned and characterized. Nucleotide sequence analysis reveals the presence of an intron within the structural gene and predicts the synthesis of a polypeptide of 448 amino acid residues. Comparison of nucleotide and amino acid sequences with the Saccharomyces cerevisiae alpha-tubulin encoding genes shows a 75% homology and about 92% similarity respectively. In contrast to S. cerevisiae, C. albicans appears to possess only one gene for alpha-tubulin which is able to functionally complement a S. cerevisiae cold-sensitive tub1 mutant.

γ-Tubulin and the fungal microtubule cytoskeleton

γ-Tubulin is present in phylogenetically diverse eukaryotes. It is a component of microtubule organizing centers such as the spindle pole bodies of fungi. In Aspergillus nidulans and Schizosaccharomyces pombe, it is essential for nuclear division, and, thus, for viability. In A. nidulans, nuclei carrying a γ-tubulin disruption can be maintained in heterokaryons, and the phenotypes caused by the disruption can be determined in uninucleate spores produced by the heterokaryons. Experiments with heterokaryons created in strains with mutations that allow synchronization of the cell cycle reveal that γ-tubulin is not required for the transition from the G1phase of the cell cycle through S phase to G2, nor for the entry into mitosis as judged by chromosomal condensation. It is, however, required for the formation of the mitotic spindle and for the successful completion of mitosis. Staining with the MPM-2 monoclonal antibody reveals that spindle pole body replication occurs in the absence of functional γ-tubulin. Finally, human γ-tubulin functions in fission yeast, and this indicates that γ-tubulin has similar functions in widely divergent organisms. Key words: tubulin, microtubule, spindle pole body, microtubule organizing center.

Sequences controlling transcription of the Chlamydomonas reinhardtii beta 2-tubulin gene after deflagellation and during the cell cycle

In Chlamydomonas reinhardtii, transcripts from the beta 2-tubulin gene (tubB2), as well as those from other tubulin-encoding genes, accumulate immediately after flagellar excision as well as at a specific time in the cell cycle. Control of tubB2 transcript accumulation following deflagellation is regulated, at least partially, at the transcriptional level. We have fused the tubB2 promoter to the arylsulfatase (ars) reporter gene, introduced this construct into C. reinhardtii, and compared expression of the chimeric gene with that of the endogenous tubB2 gene. After flagellar excision, transcripts from the tubB2/ars chimeric gene accumulate with kinetics similar to those of transcripts from the endogenous tubB2 gene. The tubB2/ars transcripts also accumulate in a cell cycle-specific manner; however, chimeric transcripts are more abundant earlier in the cell cycle than the endogenous tubB2 transcripts. To elucidate transcriptional control of tubB2, we have mutated or removed sequences in the tubB2 promoter and examined the effect on transcription. The tubB2 promoter shares features with the promoters of other tubulin-encoding genes; these include a GC-rich region between the TATA box and the transcription initiation site and multiple copies of a 10-bp sequence motif that we call the tub box. The tubB2 gene contains seven tub box motifs. Changing the GC-rich region to an AT-rich region or removing three of the seven tub box motifs did not significantly affect transcription of the chimeric gene. However, removing four or five tub box motifs prevented increased transcription following deflagellation and diminished cell cycle-regulated transcription from the tubB2 promoter.

Sequences controlling transcription of the Chlamydomonas reinhardtii beta 2-tubulin gene after deflagellation and during the cell cycle

In Chlamydomonas reinhardtii, transcripts from the beta 2-tubulin gene (tubB2), as well as those from other tubulin-encoding genes, accumulate immediately after flagellar excision as well as at a specific time in the cell cycle. Control of tubB2 transcript accumulation following deflagellation is regulated, at least partially, at the transcriptional level. We have fused the tubB2 promoter to the arylsulfatase (ars) reporter gene, introduced this construct into C. reinhardtii, and compared expression of the chimeric gene with that of the endogenous tubB2 gene. After flagellar excision, transcripts from the tubB2/ars chimeric gene accumulate with kinetics similar to those of transcripts from the endogenous tubB2 gene. The tubB2/ars transcripts also accumulate in a cell cycle-specific manner; however, chimeric transcripts are more abundant earlier in the cell cycle than the endogenous tubB2 transcripts. To elucidate transcriptional control of tubB2, we have mutated or removed sequences in the tubB2 promoter and examined the effect on transcription. The tubB2 promoter shares features with the promoters of other tubulin-encoding genes; these include a GC-rich region between the TATA box and the transcription initiation site and multiple copies of a 10-bp sequence motif that we call the tub box. The tubB2 gene contains seven tub box motifs. Changing the GC-rich region to an AT-rich region or removing three of the seven tub box motifs did not significantly affect transcription of the chimeric gene. However, removing four or five tub box motifs prevented increased transcription following deflagellation and diminished cell cycle-regulated transcription from the tubB2 promoter.

Guanine nucleotide-dependent assembly of FtsZ into filaments

FtsZ is an essential cell division protein that is localized to the leading edge of the bacterial septum in a cytokinetic ring. It contains the tubulin signature motif and is a GTP binding protein with a GTPase activity. Further comparison of FtsZ with eukaryotic tubulins revealed some additional sequence similarities, perhaps indicating a similar GTP binding site. Examination of FtsZ incubated in vitro by electron microscopy revealed a guanine nucleotide-dependent assembly into protein filaments, supporting the hypothesis that the FtsZ ring is formed through self-assembly. FtsZ3, which is unable to bind GTP, does not polymerize, whereas FtsZ2, which binds GTP but is deficient in GTP hydrolysis, is capable of polymerization.

Regulation and evolution of the single alpha-tubulin gene of the ciliate Tetrahymena thermophila

The single alpha-tubulin gene of Tetrahymena thermophila was isolated from a genomic library and shown to encode a single protein. Comparisons of the rates of evolution of this gene with other alpha-tubulin sequences revealed that it belongs to a group of more evolutionarily constrained alpha-tubulin proteins in animals, plants, and protozoans versus the group of more rapidly evolving fungal and variant animal alpha-tubulins. The single alpha-tubulin of Tetrahymena must be used in a variety of microtubule structures, and we suggest that equivalently conserved alpha-tubulins in other organisms are evolutionarily constrained because they, too, are multifunctional. Reduced constraints on fungal tubulins are consistent with their simpler microtubule systems. The animal variant alpha-tubulins may also have diverged because of fewer functional requirements or they could be examples of specialized tubulins. To analyze the role of tubulin gene expression in regulation of the complex microtubule system of Tetrahymena, alpha-tubulin mRNA amounts were examined in a number of cell states. Message levels increased in growing versus starved cells and also during early stages of conjugation. These changes were correlated with increases in transcription rates. Additionally, alpha-tubulin mRNA levels oscillate in a cell cycle dependent fashion caused by changes in both transcription and decay rates. Therefore, as in other organisms, Tetrahymena adjusts alpha-tubulin message amounts via message decay. However the complex control of alpha-tubulin mRNA during the Tetrahymena life cycle involves regulation of both decay and transcription rates.

Molecular genetic distinction of Pneumocystis carinii from rats and humans

Pneumocystis carinii from rats and from humans were compared with respect to electrophoretic karyotype, presence of DNA sequences known to be repeated in rat-derived P. carinii, overall DNA sequence homology, and the sequences at two genetic loci. The organisms from each host species were different in each respect. Neither of two repeated DNAs from rat-derived P. carinii was found in the genome of human-derived organisms, and total DNA from rat-derived P. carinii failed to hybridize to human-derived P. carinii DNA. The sequences of the alpha-tubulin genes from the two P. carinii were strikingly different and the base composition of the alpha-tubulin gene from rat-derived P. carinii was rich in adenine and thymine, while the base composition of this gene from human-derived P. carinii was rich in guanine and cytosine. The sequence from the 18S rRNA gene of human-derived P. carinii was twice as divergent from that of rat-derived P. carinii as the sequence from the corresponding region of Candida albicans was from that of Candida tropicalis. These data show that rats and humans can harbor distinct types of P. carinii that are sufficiently different to suggest that P. carinii from the two hosts could be different species.

Either alpha-tubulin isogene product is sufficient for microtubule function during all stages of growth and differentiation in Aspergillus nidulans

The filamentous fungus Aspergillus nidulans has two genes encoding alpha-tubulin, tubA and tubB, which are differentially required at distinct stages during the life cycle. The tubA gene is required during vegetative growth for mitosis and nuclear migration (B. R. Oakley, C. E. Oakley, and J. E. Rinehart, Mol. Gen. Genet. 208:135-144, 1987; P. Doshi, C. A. Bossie, J. H. Doonan, G. S. May, and N. R. Morris, Mol. Gen. Genet. 225:129-141, 1991). The tubB gene is not required for any detectable aspect of vegetative growth or asexual reproduction but is essential during sexual development prior to the first meiotic division (K. E. Kirk and N. R. Morris, Genes Dev. 5:2014-2023, 1991). In this study, we determined whether the role of each alpha-tubulin gene is to provide a specific isotype necessary for a particular microtubule function or whether either alpha-tubulin isotype, if present in sufficient quantities, can participate effectively in all types of microtubule. Strains carrying a deletion allele of tubB (tubB delta) produce no ascospores from a cross. When one copy of a plasmid containing the region upstream of the tubB gene fused to the tubA coding region was integrated into a tubB delta strain, ascosporogenesis proceeded beyond the tubB delta block and resulted in the formation of sexual spores. However, irregular numbers of spores formed in some asci during development, and the ascospores had greatly diminished viability and aberrant morphologies. These defects were nearly corrected when two additional copies of the tubA coding region were integrated into the tubB delta strain. These results indicate that the tubA alpha-tubulin isotype can form functional microtubules during sexual development in the absence of tubB protein. In a reciprocal set of experiments, we examined whether upregulation of tubB can complement the tubA4 mutation, which causes supersensitivity to benomyl during vegetative growth. When tubA4 strains integrated a plasmid containing an alcohol-inducible promoter joined to the tubB coding region and subsequently overexpressed the tubB isotype, the benomyl supersensitivity normally caused by the tubA4 allele was relieved. These results indicate that when enough tubB alpha-tubulin is supplied, strains lacking functional tubA isotype can still form microtubules which effectively carry out mitosis and nuclear migration.

Either alpha-tubulin isogene product is sufficient for microtubule function during all stages of growth and differentiation in Aspergillus nidulans

The filamentous fungus Aspergillus nidulans has two genes encoding alpha-tubulin, tubA and tubB, which are differentially required at distinct stages during the life cycle. The tubA gene is required during vegetative growth for mitosis and nuclear migration (B. R. Oakley, C. E. Oakley, and J. E. Rinehart, Mol. Gen. Genet. 208:135-144, 1987; P. Doshi, C. A. Bossie, J. H. Doonan, G. S. May, and N. R. Morris, Mol. Gen. Genet. 225:129-141, 1991). The tubB gene is not required for any detectable aspect of vegetative growth or asexual reproduction but is essential during sexual development prior to the first meiotic division (K. E. Kirk and N. R. Morris, Genes Dev. 5:2014-2023, 1991). In this study, we determined whether the role of each alpha-tubulin gene is to provide a specific isotype necessary for a particular microtubule function or whether either alpha-tubulin isotype, if present in sufficient quantities, can participate effectively in all types of microtubule. Strains carrying a deletion allele of tubB (tubB delta) produce no ascospores from a cross. When one copy of a plasmid containing the region upstream of the tubB gene fused to the tubA coding region was integrated into a tubB delta strain, ascosporogenesis proceeded beyond the tubB delta block and resulted in the formation of sexual spores. However, irregular numbers of spores formed in some asci during development, and the ascospores had greatly diminished viability and aberrant morphologies. These defects were nearly corrected when two additional copies of the tubA coding region were integrated into the tubB delta strain. These results indicate that the tubA alpha-tubulin isotype can form functional microtubules during sexual development in the absence of tubB protein. In a reciprocal set of experiments, we examined whether upregulation of tubB can complement the tubA4 mutation, which causes supersensitivity to benomyl during vegetative growth. When tubA4 strains integrated a plasmid containing an alcohol-inducible promoter joined to the tubB coding region and subsequently overexpressed the tubB isotype, the benomyl supersensitivity normally caused by the tubA4 allele was relieved. These results indicate that when enough tubB alpha-tubulin is supplied, strains lacking functional tubA isotype can still form microtubules which effectively carry out mitosis and nuclear migration.

Cloning and characterization of an alpha-tubulin-encoding gene from rat-derived Pneumocystis carinii

The gene and cDNA (TUB1) encoding an alpha-tubulin (alpha-Tub) from Pneumocystis carinii carried in rats were isolated and sequenced. The gene produced a 1.4-kb transcript that contained an open reading frame of 1347 nucleotides coding for a protein of 50,326 Da. The nt sequence of TUB1 of P. carinii was 65-71% identical to other TUB1 genes, including those of yeast and human. The deduced amino acid sequence of P. carinii alpha-Tub showed 75-84% identity to other alpha-Tubs. Eight small introns resided within TUB1. P. carinii TUB1 was mapped to a 425-kb chromosome. The cloned gene appears to be the only alpha-Tub-encoding gene in the P. carinii genome.

Cell division in Aspergillus

Amenable to sophisticated genetic and molecular analysis, the simple filamentous fungus Aspergillus nidulans has provided some novel insights into the mechanisms and regulation of cell division. Mutational analysis has identified over fifty genes necessary for nuclear division, nuclear movement and cytokinesis. Molecular and cellular analysis of these mutants has led to the discovery of novel components of the cytoskeleton as well as to clarifying the role of established cytoskeletal proteins. Mutations leading to defects in the kinases (i.e. p34cdc2) and phosphatases (i.e. cdc25 and PP1), which are known to regulate mitosis in other eukaryotes, have been identified in Aspergillus. Additional, as yet novel, mitotic regulatory molecules, encoded by the nimA and bimE genes, have also been discovered in Aspergillus.

Cloning, sequence and expression of a beta-tubulin-encoding gene in the homobasidiomycete Schizophyllum commune

The beta-tubulin (beta Tub)-encoding gene (tub-2) of Schizophyllum commune is the first tubulin gene isolated, cloned and sequenced from higher filamentous fungi (homobasidiomycetes). The S. commune tub-2 gene is organized into nine exons and eight introns. The introns vary from 48 to 107 nt in length, and are distributed throughout the gene. The tub-2 exons code for a protein of 445 amino acids (aa), which shows great homology with beta Tubs of filamentous ascomycetes, plants, and animals, but less homology with yeasts. The codon usage of tub-2 from S. commune is biased, as it is in most beta Tub-encoding genes of filamentous fungi. The S. commune beta Tub shows a conserved aa sequence in the C-terminal domain, which is suggested to interact with microtubule-associated proteins in animals. In contrast, the S. commune beta Tub deviates from most known beta Tubs by having a Cys165 residue, which might be significant for the insensitivity of S. commune haploid strains to the antimicrotubule drug, benomyl. In tub-2 of different haploid strains, sequence polymorphisms occur in the 5' and 3' flanking regions. The expression of tub-2 is high in young mycelium, which has a high number of extending apical cells, but decreases with the aging of the mycelium. No significant difference in the hybridization signal intensity for the tub-2 transcripts was recorded either during intercellular nuclear migration at early mating, or in mycelia with a mutation in the B mating-type gene.(ABSTRACT TRUNCATED AT 250 WORDS)

Tubulin structure and biochemistry

In the past year, much has been learned about structure-function correlations in the tubulin molecule, and specifically about the nature and roles of post-translational modifications and tubulin isotypes. The interactions between tubulin and its ligands--both microtubule-associated proteins and anti-mitotic drugs--are becoming clearer at the molecular level.

Mitotic gold in a mold: Aspergillus genetics and the biology of mitosis

The analysis of fungal mutants has had an extraordinary impact on our understanding of the biochemistry and regulation of mitosis. In this article we review the contribution of work on the filamentous fungus Aspergillus nidulans to the molecular genetics of mitosis.

The tubB alpha-tubulin gene is essential for sexual development in Aspergillus nidulans

The filamentous fungus Aspergillus nidulans has two genes encoding alpha-tubulin, tubA and tubB. Mutational analysis of tubA has demonstrated that the tubA gene is essential for mitosis and nuclear migration. In this study we have deleted the tubB gene by replacing it with a selectable marker and have named this new allele tubB delta. The results demonstrate that the tubB gene is not required for vegetative growth or asexual reproduction, nor is it required for the initiation or early stages of sexual differentiation. Deletion of tubB, however, completely prevents ascosporogenesis, because tubB delta strains produce no sexual spores when self-crossed. These strains produce viable ascospores when outcrossed to tubB+ strains, indicating that the tubB delta mutation is recessive. We have studied the cytology of sexual development in wild-type strains and in the tubB mutant and have observed that tubB delta. strains develop normally to the stage of ascus formation. However, only a single nuclear mass is observed in the tubB delta ascus, indicating that either the two zygotic haploid nuclei are blocked in karyogamy or that karyogamy occurs but the resulting diploid nucleus is subsequently blocked in meiosis I.