Genetics and biochemistry of morphogenesis in Neurospora

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.

Lessons from the genome sequence of Neurospora crassa: tracing the path from genomic blueprint to multicellular organism

We present an analysis of over 1,100 of the approximately 10,000 predicted proteins encoded by the genome sequence of the filamentous fungus Neurospora crassa. Seven major areas of Neurospora genomics and biology are covered. First, the basic features of the genome, including the automated assembly, gene calls, and global gene analyses are summarized. The second section covers components of the centromere and kinetochore complexes, chromatin assembly and modification, and transcription and translation initiation factors. The third area discusses genome defense mechanisms, including repeat induced point mutation, quelling and meiotic silencing, and DNA repair and recombination. In the fourth section, topics relevant to metabolism and transport include extracellular digestion; membrane transporters; aspects of carbon, sulfur, nitrogen, and lipid metabolism; the mitochondrion and energy metabolism; the proteasome; and protein glycosylation, secretion, and endocytosis. Environmental sensing is the focus of the fifth section with a treatment of two-component systems; GTP-binding proteins; mitogen-activated protein, p21-activated, and germinal center kinases; calcium signaling; protein phosphatases; photobiology; circadian rhythms; and heat shock and stress responses. The sixth area of analysis is growth and development; it encompasses cell wall synthesis, proteins important for hyphal polarity, cytoskeletal components, the cyclin/cyclin-dependent kinase machinery, macroconidiation, meiosis, and the sexual cycle. The seventh section covers topics relevant to animal and plant pathogenesis and human disease. The results demonstrate that a large proportion of Neurospora genes do not have homologues in the yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe. The group of unshared genes includes potential new targets for antifungals as well as loci implicated in human and plant physiology and disease.

The product of the het-C heterokaryon incompatibility gene of Neurospora crassa has characteristics of a glycine-rich cell wall protein

Filamentous fungi are capable of hyphal fusion, but heterokaryon formation between different isolates is controlled by specific loci termed het loci. Heterokaryotic cells formed between strains of different het genotype are rapidly destroyed or strongly inhibited in their growth. In Neurospora crassa, at least 11 loci, including the mating type locus, affect the capacity to form a heterokaryon between different isolates. In this report, we describe the molecular characterization of the vegetative incompatibility locus, het-C. The het-COR allele was cloned by genetically identifying the het-C locus in a chromosome walk, and the activity of clones containing the het-COR allele was tested in a functional transformation assay. The het-COR allele encodes a 966-amino acid polypeptide with a putative signal peptide, a coiled-coil motif and a C-terminal glycine-rich domain, similar to glycine-rich domains detected in various extracellular and structural cell envelope proteins. Both the coiled-coil and one-third of the glycine-rich carboxyl terminal domains were required for full het-COR activity. Mutants of het-COR were obtained by repeat-induced point mutation (RIP); these mutants were indistinguishable from wild type during vegetative growth and sexual reproduction but displayed dual compatibility with both of two mutually incompatible het-COR and het-cPA strains.

Hyphal development in Neurospora crassa: involvement of a two-component histidine kinase

Two-component signal transduction systems are most often found in prokaryotic organisms where they are responsible for mediating the cellular responses to many environmental stimuli. These systems are composed of an autophosphorylating histidine kinase and a response regulator. We have found evidence for the existence of two-component histidine kinases in the eukaryotic filamentous fungus Neurospora crassa based on screening with degenerate primers to conserved regions of these signaling proteins. Subsequent cloning and sequencing of one member of this newly discovered group, nik-1+, shows that the predicted protein sequence shares homology with both the kinase and response regulator modules of two-component signaling proteins. In addition, the N-terminal region of the protein has a novel repeating 90-amino acid motif. Deletion of the nik-1+ gene in N. crassa results in an organism that displays aberrant hyphal structure, which is enhanced under conditions of high osmostress. Increased osmotic pressure during growth on solid medium leads to restricted colonial growth, loss of aerial hyphae formation, and no subsequent conidiophore development. This finding may have implications for mechanisms of fungal colonization and pathogenicity.

Inactivation of the Podospora anserina vegetative incompatibility locus het-c, whose product resembles a glycolipid transfer protein, drastically impairs ascospore production

The het-c locus contains different alleles that elicit nonallelic vegetative incompatibility through specific interactions with alleles of the unlinked loci het-e and het-d. The het-c2 allele has been cloned. It encodes a 208-amino acid polypeptide that is similar to a glycolipid transfer protein purified from pig brain. Disruption of this gene drastically impairs ascospore production in homozygous crosses, and some mutants exhibit abnormal branching of apical hyphae. The protein encoded by het-c2 is essential in the biology of the fungus and may be involved in cell-wall biosynthesis.

Pleiotropic and differential phenotypic expression of two sn (snowflake) mutant alleles of Neurospora crassa: analysis in homokaryotic and heterokaryotic cells

Mutations sn (snowflake) JL301 and C136, in the centromere region of linkage group I in Neurospora crassa, are at 0.6-3.0 map units to the left of the os-4 locus. Strains carrying snJL301 produce very short aerial hyphae and only arthroconidia, and do not grow in high salt media. snC136 strains produce aerial hyphae, with abnormally large and rounded blastoconidia, at the top of the agar slant cultures, and revert to wild-type growth in high salt media. Studies with forced primary heterokaryons indicate that snJL301 is recessive while snC136 is a semi-dominant and gene-dose dependent allele, with respect to the wild-type. Taken together the results show that: (1) the sn mutations are allelic with a differential pleiotropic phenotype, and (2) snC136 may code for a partially functioning gene product while snJL301 appears to be a null allele.

In vitro inhibition of stable 1,3-beta-D-glucan synthase activity from Neurospora crassa

Glucan synthase activity of Neurospora crassa was isolated by treatment of protoplast lysates with 0.1% 3-[(3-cholamidopropyl)-dimethylammonio]-1-propanesulfonate and 0.5% octylglucoside in 25 mM 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid buffer, pH 7.4, containing 5 mM EDTA, 1 mM phenylmethylsulfonylfluoride, 200 mM inorganic phosphate, 10 microM GTP, 1 mM DTT, 10 mM sodium fluoride, and 600 mM glycerol. Resulting activity was partially purified by sucrose gradient density sedimentation. Approximately 70% of enzyme activity in the sucrose gradient peak fraction was soluble and enzyme activity was purified 7.3-fold. Partially purified enzyme activity had a half-life of several weeks at 4 degrees C, and a Km(app) of 1.66 +/- 0.28 mM. Inhibitors (Cilofungin, papulacandin B, aculeacin A, echinocandin B, sorbose and UDP) of 1,3-beta-D-glucan synthase activity were tested against crude particulate and detergent treated enzyme fractions and the Ki(app) of each inhibitor determined. It seems likely that this stable preparation of glucan synthase activity may be useful for in vitro enzyme screens for new glucan synthase inhibitors.

Genetic control of a structural polymer of the Neurospora crassa cell wall

The heteropolysaccharide present in fraction 1 of the Neurospora crassa cell wall has been characterized in wild-type and morphological mutant strains of this fungus. Single and double mutations have been studied to determine possible genetic interactions controlling the chemical composition of such heteropolysaccharides . Single mutations studied were peak-2, scumbo ( FGSC 49), ragged ( FGSC 296), and crisp -1 ( FGSC 488). Double mutations studied were peak-2, scumbo ( FGSC 419), and ragged crisp -1. In all these strains, the main constituents of the heteropolysaccharide were glucose, mannose and galactose. Glycosidic linkages binding these neutral sugars have been identified by gas-liquid chromatography. A chemical structure of fraction I heteropolysaccharide is proposed. The results obtained with double mutants suggest the existence of genetic interactions, such as complementation or additive effects of lesions of different genes, to control the chemical composition and structure of the cell wall and the morphology of N. crassa mycelium.

Pleiotropic effects of ribosomal mutations for cycloheximide resistance in a double-resistant homocaryon of Neurospora crassa

Pleiotropic effects of two ribosomal mutations for cycloheximide resistance were studied in a double-resistant homocaryon. The results obtained indicated that the combination of the two ribosomal mutations results in: (i) morphological abnormalities which suggest a severe distortion in the extension of cell walls and membranes; (ii) disturbance in the normal 60S-40S subunit ratio; (iii) decreased rate of cell mass production not necessarily associated with inbalance in the 60S-40S ratio; and (iv) cold sensitivity which does not interfere with mycelial mass production.

Characterization of the carbohydrate component of fraction I in the Neurospora crassa cell wall

The carbohydrate portion of fraction I of the Neurospora crassa cell wall has been analyzed for sugar composition by gas-liquid chromatography and colorimetric methods. The analysis was performed comparatively in a wild-type strain (RL 3-8A) and three morphological mutants: scumbo (FGSC 49), peak-2a (a mutant known to be allelic to biscuit), and ragged (FGSC 296). Fraction I of all strains studied contains glucose, mannose, and galactose as the main sugars. Uronic acids and amino sugars are also present in small amounts. The glycosidic linkages binding the neutral sugars were analyzed by Lindberg's combined gas chromatography-mass spectrometry techniques for identification of the partially methylated alditol acitate sugar derivatives. The main polymeric portion of fraction I seems to be a linear glucan with the glucose residues linked by 1 leads to 3 and 1 leads to 4 bonds. A mannan portion with a branched configuration is also present, with galactose as the sugar residue which serves as branches in the molecule(s). The branched mannan portion appears to increase in amount in correlation with more drastic morphological changes of the mycelia. In this respect, the mutant ragged has the lowest mycelial growth rate and the largest amount of mannan. The importance of the polysaccharide structure of fraction I on the colonial morphology of the mycelia is discussed.

Isolation and characterization of glycerol-3-phosphate dehydrogenase-defective mutants of Neurospora crassa

Three glycerol-nonutilizing mutants deficient in the mitochondrial glycerol-3-phosphate (G3P) dehydrogenase (EC 1.1.99.5) were isolated from inl(ts) derivatives of Neurospora crassa following inositolless death at elevated temperatures on minimal glycerol medium. These mutants failed to grow on glycerol as a sole carbon source, but could grow on acetate, glucose, or mannitol media and were female fertile in genetic crosses, thereby distinguishing them from the previously reported polyol-protoperithecial defective Neurospora mutants. In addition, these glp mutants exhibited a distinct morphological alteration during vegetative growth on sucrose slants and colonial growth on sorbose-containing semicomplete medium. The glp-2 locus was assigned a location between arg-5 and nuc-2 on chromosome IIR on the basis of two-factor crosses and by duplication coverage by insertional translocation ALS176, but not NM177. All mutations were allelic as judged from the absence of both complementation in forced heterokaryons and genetic recombination among glp-2 mutations. The reversion frequency of all three mutations was less than 10(10), indicating probable deletions in these strains. No G3P dehydrogenase activity could be detected in either cytosolic or mitochondrial extracts from mutant strains grown on glycerol, glucose, or galactose media. These results suggest that the glp-2 locus may be the structural gene for both the cytosolic and mitochondrial forms of G3P dehydrogenase or for a cytosolic precursor of the mitochondrial G3P dehydrogenase. The defect is specific for the G3P dehydrogenase since normal activities of the mitochondrial cytochrome oxidase and succinate dehydrogenase and the cytosolic glycerol dehydrogenase and dihydroxyacetone phosphate reductase are detected in mutant extracts. During attempted growth of glp-2 mutants on glycerol media, there was an accumulation of G3P in culture filtrates, a reduction in the mycelial growth rate, and a decreased level of glycerokinase induction.