Dehydrogenases of neurospora crassa

Isolation and properties of 6-phosphogluconate dehydrogenase from Escherichia coli. Some comparisons with the thermophilic enzyme from Bacillus stearothermophilus

6-Phosphogluconate dehydrogenase (6-phospho-D-gluconate:NADP oxidoreductase (decarboxylating), EC 1.1.1.44) of Escherichia coli MREp 600 has been isolated with the purpose of carrying out comparative studies with the thermostable enzyme previously isolated from Bacillus stearothermophilus (Veronese, F.M., Boccù, E.,Fontana, A., Benassi,C.A., and Scoffone, E. (1974), Biochim, Biophys. Acta 334, 31). The purified enzyme appeared homogeneous by the criteria of disc gel electrophoresis with and without sodium dodecyl sulfate, ultracentrifugation, and gel filtration. The enzyme has enzymological and physiochemical properties similar to the enzyme isolated from other sources, including B. stearothermophilus. The E. coli enzyme has a mol wt of 100,000 +/- 3000 and is composed of two apparently identical subunits. The amino acid composition of both the mesophilic and thermophilic enzyme has been determined and found to present large similarities. The E. coli enzyme shows a high degree of specificity for nicotinamide adenine dinucleotide (NADP) and it is inhibited by reduced NADP (NADPH). Cysteine residues are involved in the catalytic activity, since on incubation of the enzyme with p-chloromercuribenzoate or 5.5'-dithiobis(2-nitrobenzoic acid) strong inhibition occurs, activity being restored by treatment with excess of beta-mercaptoethanol. The substrate 6-phosphogluconate protects partially the enzyme from inactivation. Both the mesophilic and thermophilic 6-phosphogluconate dehydrogenases are inactivated by Rose Bengal in the presence of light by similar kinetics and protected against photoinactivation by the enzyme substrate. The E. coli enzyme, on the other hand, showed distinct differences in stability against heat and unfolding agents in respect to the B. stearothermophilus enzyme. Heating at 50 degrees C or incubation in 8 M urea results in rapid inactivation. The gross structure of the mesophilic and thermophilic enzyme was very similar as judged by circular dichroic measurements. The far-ultraviolet circular dichroic spectrum had a negative band centered at about 220 nm. In both cases, the fluorescence emission spectrum indicates that the environment of the tryptophan residues is similar, since both enzymes show an emission maximum at 334 nm upon excitation at 295 nm. Circular dichroism measured at various temperatures between 25 and 80 degrees C showed the mesophilic enzyme to be conformationally stable below about 45 degrees C and the thermophilic enzyme below 60 degrees C. The secondary structure of the E. coli enzyme was very sensitive to the denaturing action of urea, since in 8 M urea it rapidly unfolded. Partial renaturation after urea treatment occurred on dilution with buffer or dialysis, as evidenced by spectral properties of the renatured enzyme. The results show that the mesophilic and thermophilic enzymes are very similar and that differences in thermal stability depend on subtle differences in the architectures of the proteins.

A GENETIC STUDY OF ELECTROPHORETICALLY VARIANT EXTRACELLULAR AMYLOLYTIC ENZYMES OF WILD-TYPE STRAINS OF ASPERGILLUS NIDULANS

Ninety strains assigned to Aspergillus nidulans produced extracellular amylolytic enzymes in a defined medium containing soluble starch as the organic carbon source. Acrylamide gel electrophoresis of freeze-dried culture filtrates gave 6-9 sites of amylolytic activity accommodated by nine patterns. Eighty-one strains exhibited pattern 1 or 2 and not more than two displayed one of the remaining seven patterns. Crosses between strains with different patterns succeeded only for strains with pattern 1 or 2. Esterase and phosphatase zymograms for mycelial extracts indicated that strains with amylase pattern 1 or 2 are A. nidulans, while the strains with the other amylase patterns are probably cryptic or sibling species. Two codominant alleles are responsible for the different electrophoretic mobility of one pair of amylolytic sites in zone 2 which distinguishes patterns 1 and 2. The pair of sites may be determined by duplicate loci.

Glucose-6-phosphate dehydrogenase and Neurospora morphology

In addition to the Neurospora crassa mutant col-2, mutations in two other unlinked genes, balloon and frost, lead to distinct morphological growth and abnormal glucose-6-phosphate dehydrogenases. The glucose-6-phosphate dehydrogenases from these strains have increased thermolabilities, altered kinetic parameters, and distinctive electrofocusing patterns. The altered enzyme segregates with each morphological phenotype in crosses to wild type, and there is a correlation between the severity of the enzymic defect and the degree of morphological abnormality in heterocaryons and double mutants. These results are compatible with the hypotheses that the balloon, frost, and col-2 genes are structural genes for glucose-6-phosphate dehydrogenase and that the pleiotropic effects of these altered dehydrogenases are responsible for the mutant morphologies.