Influence of the carbon source on glycerol kinase activity in Neurospora crassa

Genetic and enzymatic characterization of the inducible glycerol dissimilatory system of Neurospora crassa

The glycerol dissimilatory system in Neurospora crassa was analyzed through the characterization of 18 Glp- mutants which were isolated after inositol-less death and filtration enrichment. All mutants obtained by this procedure could be assigned to one of three complementation groups. The subsequent genetic characterization of these glp mutations revealed lesions on the I, II, and VI chromosomes at the glp-1, glp-2, and glp-4 loci, each of which was subjected to fine-structure analysis. Evidence from the enzymatic characterization of these mutants indicated that glp-2 and glp-4 were the structural genes encoding the mitochondrial glycerol-3-phosphate dehydrogenase and cytosolic glycerokinase, respectively. Additional evidence, obtained from studies of the inducibility of glycerokinase by glycerol, cold treatment, or deoxyribose, suggests that glp-1 is involved in controlling the expression of glp-4.

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.

Characterization of glycerol nonutilizing and protoperithecial mutants of Neurospora

Mutants defective in polyol metabolism and/or in protoperithecial development were selected in Neurospora tetrasperma, a species in which protoperithecial development occurs at nonpermissively high temperature if certain polyols are used in lieu of sucrose as carbon source. Mutants selected for nonutilization of one of the four polyols tested, glycerol, mannitol, sorbitol, or xylitol, were usually found to be nonutilizers of the other three polyols as well. Mutants blocked at various stages of protoperithecial development complemented pairwise to produce more advanced developmental stages, usually mature protoperithecia and, when of opposite mating type, mature perithecia. About one-third of the mutants manifested both polyol auxotrophy and defective protoperithecial development upon initial isolation, but protoperithecial defectiveness in such mutants usually showed erratic segregation in crosses and/or instability to repeated vegetative transfer, whereas polyol auxotrophy usually did not and was, therefore, studied further. Two glycerol nonutilizing strains were introgressed into N. crassa to facilitate genetic analysis. One, glp-4, lacked both inducible and constitutive glycerol kinase and mapped to linkage group VI, between ad-1 and rib-1; the other, glp-5, lacked glyceraldehyde kinase and mapped to linkage group I, proximal to ad-9. Another mutant, gly-u(234), has been reported by other investigators to lack inducible glycerol kinase but to map to linkage group I, distal to ad-9.

Increase of enzyme activities in Neurospora crassa during incubation at low temperatures

The effect of lowering the incubation temperature of sucrose-grown cultures of Neurospora crassa on the level of various enzyme activities was investigated. Of twelve inducible/derepressible activities studied, three, in addition to glycerol kinase, were found to increase during 48 h of incubation at 4-6 degrees C: trehalase (increase in specific activity of 3-10-fold), beta-glucosidase (6-12-fold) and beta-N-acetylglucosaminidase (4 to 6-fold). The maximum increases occurred at 6 degrees C and no increases took place in mycelia incubated at 0 degrees C. The kinetics of the changes in activity were markedly different from those observed previously with glycerol kinase. The increases were inhibited by cycloheximide. Trehalase, beta-glucosidase and beta-N-acetylglucosaminidase activities were not rapidly lost when cultures incubated at 6 degrees C were returned to 26 degrees C.