Gene-enzyme relationships in Neurospora invertase

Light regulation of asexual development in the rice blast fungus, Magnaporthe oryzae

Light is a major environmental factor that influences many biological processes. We characterized the roles of light in asexual development (including the formation of aerial hyphae and conidiophore) in Magnaporthe oryzae, which is the causal agent of rice blast disease. Our data revealed a complex nature of light regulation in the asexual developments of M. oryzae. Asexual development of M. oryzae is suppressed by blue light in a light/dark cycling environment and asexual spore release is controlled by both blue and red light. We demonstrated that even very dim light, about 10 micromol m(-2), is sufficient to suppress spore-release behavior in M. oryzae. We also generated knockout strains of a blue light receptor, mgwc-1, the M. oryzae homolog of white collar-1 in Neurospora crassa, and demonstrated blue-light-specific regulation in the asexual development and spore release in M. oryzae. Our findings in this agriculturally important pathogen, M. oryzae, broaden our understanding of the roles of light in fungal development.

Regulation of invertase in Aspergillus nidulans: effect of different carbon sources

Aspergillus nidulans produces an extracellular beta-D-fructofuranoside fructohydrolase (invertase) when grown on a medium containing the beta-fructofuranosides sucrose or raffinose, indicating that synthesis is subject to induction by the substrate. On a growth medium containing sucrose, production was maximal at 15 h in cultures incubated at 28 C degrees. After this time the level of detectable invertase in the cultures declined. A proportion of the enzyme was secreted during the linear growth phase of the fungus. Various sugars were investigated for induction of invertase, but only the two beta-fructofuranosides induced high production levels; with the other sugars, the enzyme was produced only at a low constitutive level. Mycelium grown under repressive conditions (1% glucose), rapidly produced invertase when transferred to sucrose-containing medium. After 80 min the invertase level in these cultures was 26-fold higher than the constitutive level. The repressive effect of other sugars, e.g. glucose and xylose, on invertase production was also demonstrated in this experimental system.

Molecular cloning and expression in Saccharomyces cerevisiae and Neurospora crassa of the invertase gene from Neurospora crassa

A plasmid (named pCN2) carrying a 7.6 kb BamHI DNA insert was isolated from a Neurospora crassa genomic library raised in the yeast vector YRp7. Saccharomyces cerevisiae suco and N. crassa inv strains transformed with pNC2 were able to grow on sucrose-based media and expressed invertase activity. Saccharomyces cerevisiae suco (pNC2) expressed a product which immunoreacted with antibody raised against purified invertase from wild type N. crassa, although S. cerevisiae suc+ did not. The cloned DNA hybridized with a 7.6 kb DNA fragment from BamHI-restricted wild type N. crassa DNA. Plasmid pNC2 transformed N. crassa Inv- to Inv+ by integration either near to the endogenous inv locus (40% events) or at other genomic sites (60% events). It appears therefore that the cloned DNA piece encodes the N. crassa invertase enzyme. A 3.8 kb XhoI DNA fragment, derived from pNC2, inserted in YRp7, in both orientation, was able to express invertase activity in yeast, suggesting that it contains an intact invertase gene which is not expressed from a vector promoter.

Deoxyglucose-resistant mutants of Neurospora crassa: isolation, mapping, and biochemical characterization

Neurospora crassa mutants resistant to 2-deoxyglucose have been isolated, and their mutations have been mapped to four genetic loci. The mutants have the following characteristics: (i) they are resistant to sorbose as well as to 2-deoxyglucose; (ii) they are partially or completely constitutive for glucose transport system II, glucamylase, and invertase, which are usually repressed during growth on glucose; and (iii) they synthesize an invertase with abnormal thermostability and immunological properties, suggesting altered posttranslational modification. All of these characteristics could arise from defects in the regulation of carbon metabolism. In addition, mutants with mutations at three of the loci lack glucose transport system I, which is normally synthesized constitutively by wild-type N. crassa. Although the basis for this change is not yet clear, the mutants provide a way of studying the high-affinity system II uncomplicated by the presence of the low-affinity system I.

Isolation and characterization of Neurospora mutants affected in invertase synthesis

We have outlined a procedure that allows the large-scale screening of mutagenized Neurospora crassa populations for invertaseless mutants. We have isolated and characterized three mutations, inv(DBL1), inv(DBL9) and inv(DBL14), which have been mapped at or near the invertase structural gene. One of these, inv(DBL1), is particularly interesting. Our experiments indicate that the reduced level of invertase activity in the inv(DBL1)-containing cell can be explained as the result of a reduced number of normal enzyme molecules. We also show that wild-type Neurospora is able to respond rapidly to a change of medium and can dramatically increase its production of invertase within 20 min after a transfer to a carbon-free medium.

Mutants of Neurospora deficient in nicotinamide adenine dinucleotide (phosphate) glycohydrolase

A new screening technique has been developed for the rapid identification of Neurospora crassa mutants that are deficient in nicotinamide adenine dinucleotide glycohydrolase (NADase) and nicotinamide adenine dinucleotide phosphate glycohydrolase (NADPase) activities. Using this procedure, five single-gene mutants were isolated whose singular difference from wild type appeared to be the absence of NAD(P)ase (EC 3.2.2.6). All five mutants were found to be genetically allelic and did not complement in heterocaryons. This gene, nada [NAD(P)ase], was localized in linkage group IV. One of the nada alleles was found to specify an enzyme that was critically temperature sensitive and had altered substrate affinity. Mutations at the nada locus did not affect the genetic program for the expression of NAD(P)ase during cell differentiation, nor did they have a general effect on NAD catabolism. Nada mutations did not have simultaneous effects on other glycohydrolase activities. Tests of dominance (in heterocaryons) and in vitro mixing experiments did not provide evidence that nada mutations alter activators or inhibitors of NAD(P)ase. Thus, the nada gene appears to specify only the structure of N. crassa NAD(P)ase.

Biochemical and histochemical localization of invertase in Neurospora crassa during conidial germination and hyphal growth

The intracellular localization of Neurospora invertase, an enzyme partially secreted and partially retained by Neurospora at the cell periphery, was investigated. A cell wall fraction was isolated, to which 24% of the cell-bound invertase was firmly attached. A sensitive osmiophilic stain for invertase was developed and used in conjunction with the technique of indirect immunofluorescence to follow the pattern of invertase localization during the development of Neurospora from the germination of conidia to the mature hypha. These studies revealed that: (i) conidial invertase was uniformly distributed along the cell periphery; (ii) growing hyphal tips of germinating conidia showed pronounced invertase activity as the rest of the conidial cell wall lost its peripheral activity; (iii) hyphae in early log-phase growth had strong enzyme activity associated with the cell wall, and in late log phase the activity became associated with the plasma membrane and points where new hyphal branches were being formed; and (iv) hyphae in early stationary phase had strong fluorescence at incipient branching points, in "dots" close to the plasma membrane, and in the cytoplasm.

Genetic determinants of circadian rhythmicity in Neurospora

Timex, a strain of Neurospora crassa which exhibits a circadian rhythm of conidia formation in growth-tube cultures, has been found to differ from wild-type strains by two genes. One gene, inv, is responsible for an invertase deficiency, whereas the second gene, bd, is of unknown function. Both genes map independently from other genes known to induce Neurospora rhythmicity. The inv gene is not essential for the timex phenotype because bd strains express that phenotype on certain media. Although inv strains do exhibit some rhythmicity of their own, the rhythmicity apparently is not a direct result of the invertase deficiency, since there is no correlation between invertase level and rhymicity in 29 strains tested. Of the 29 strains tested, 20 exhibited some rhythmicity in growth-tube cultures, suggesting that morphological manifestations of rhythmicity in Neurospora may result from the function or the loss of function of numerous genes, or both. There was no correlation in these strains between rhythmicity and (i) genetic background; (ii) geographical origin; or (iii) nutritional requirements.