Effects of pH and temperature on the wild-type and a mutant form of Neurospora glutamate dehydrogenase

1. We confirm the observation of Bürk (1965) that Neurospora crassa NADP-linked glutamate dehydrogenase normally exists in an inactive form below pH7.0 and in a fully active form above pH8.0 in either tris or orthophosphate buffer. At pH7.4 the enzyme is about half activated at 25 degrees . 2. The variety of the enzyme produced by the mutant am(2l) shows a similar behaviour except that the transition is shifted about one pH unit in the alkaline direction. 3. The am(2l) enzyme has previously been reported to be activated by brief warming to 30 degrees in phosphate buffer at pH8.0. The wild-type enzyme shows a similar effect at pH7.0. In tris buffer this effect is much less pronounced. 4. The am(2l) enzyme is extremely unstable at 47 degrees at pH7.0; its stability is somewhat greater at lower pH, and is markedly increased by increasing the pH in the range 7.0-8.7. The wild-type enzyme also shows an indication of a stability minimum at pH7.0, but a temperature of 60 degrees is needed for a measurable rate of inactivation. 5. The inactive form of the enzyme is much more subject to thermal irreversible denaturation than is the active form.

Altered protein formation as a result of suppression in Neurospora crassa

The mutant td201 of Neurospora crassa is mutated in the trp-3 locus and forms an altered tryptophan synthetase. A suppressor mutation, su2-6, in this mutant, unlinked to the trp-3 locus, results in the production of wild-type tryptophan synthetase activity, which accounts for the alleviation of the tryptophan or indole requirement. This enzyme activity is associated with a protein physically dissimilar to the wild-type enzyme. A second altered protein, a serologically cross-reacting material is also formed in the suppressed mutant, in addition to the altered enzyme normally formed by the td201 mutant. Normal growth, equivalent to that of wild type, is not restored in the suppressed mutant even with tryptophan supplementation. The relationship of the data to possible mechanisms of suppression is discussed.

Evidence for multiple forms of isocitrate lyase in Neurospora crassa

The effect of carbon source on isocitrate lyase formation was studied in a wildtype strain of Neurospora crassa and in a uridine-deficient mutant. A constitutive level of the enzyme was produced in a casein hydrolysate medium. The enzyme was repressed by glucose, although the two strains varied with respect to the degree of glucose repression. Acetate strongly stimulated isocitrate lyase formation. The enzyme formed in the presence of acetate differed in several respects from that formed in glucose-grown cells. Differences were found in pH-activity curves, K(m) values, and in sensitivity to phosphoenolpyruvate inhibition. Diethylaminoethyl cellulose chromatography allowed separation of two enzymatically active components which showed different rates of heat inactivation. These data indicate the presence of multiple forms of isocitrate lyase in Neurospora.

Comparative inductive responses of two beta-galactosidases of Neurospora

The activities of the two beta-galactosidases of Neurospora increased in response to the presence of lactose or galactose. d-Xylose preferentially induced higher levels of activity in one of these enzymes. The initial period of the inductive response was examined in cultures transferred from noninducing to inducing media. Variations in the ratio of activities of these enzymes during this induction period indicate that their induction was not coordinate. The induction of the beta-galactosidases of Neurospora was less sensitive to low concentrations of inducers than was the induction of the beta-galactosidase of bacteria; the time lag of induction was greater in the eucaryotic organism, and the maximal induction of active enzyme was less than that observed in bacteria.

The purification and properties of histidinol dehydrogenase from Neurospora crassa

1. A procedure is described for the purification of l-histidinol dehydrogenase (l-histidinol-NAD oxidoreductase, EC 1.1.1.23) from Neurospora crassa. 2. The enzyme, as purified, has a sedimentation coefficient, S(20), of 7.1s and a molecular weight of 81 000. Considerable variation is possible in the state of polymerization of the enzyme, giving rise to observed molecular weights from 40 000 to 240 000. 3. Several kinetic parameters of the enzyme have been determined. The enzyme is maximally active at pH9.8; the K(m) (NAD) is 13.0x10(-5)m and K(m) (histidinol) is 8.2x10(-6)m. The enzyme is highly specific, does not oxidize a range of amino alcohols and other aliphatic alcohols nor reduce NADP and has no demonstrable affinity for histidine. The turnover number is 49 moles of NAD reduced/min./mole of enzyme (mol.wt. 40 000).