Genetically determined multiple forms of glutamic dehydrogenase in Neurospora crassa

Complementation in balanced heterokaryons and heterozygous diploids of Aspergillus nidulans

1. Two total and five leaky sorbitol mutants isolated in Aspergillus nidulans by defective growth on the sugar are all recessive. The mutants are closely linked, they appear to represent three linked genes spanned by a deletion.

2. Mutants which complement in heterozygous diploids do not complement in balanced heterokaryons. Failure to complement is a property of the mutants and not the result of a nutritional interaction or an unfavourable nuclear ratio in the heterokaryons.

3. Sorbitol is oxidized by an inducible enzyme system in the wild-type. There are at least two enzymes concerned in the oxidative assimilation of sorbitol, an initial oxidative enzyme, which is defective in the leaky mutants, and a later enzyme defective in the total mutants. There may also be a second non-oxidative pathway for sorbitol metabolism.

4. In diploids complementary pairs of mutants oxidized sorbitol at 75% the rate of the wild-type but non-complementary mutants did not oxidize the sugar. In balanced heterokaryons none of the pairs of mutants oxidized the substrate. It is concluded that failure of inter-genic complementation in the heterokaryons is the result of a failure of either enzyme formation or enzyme function. Models to account for differences in enzyme formation in heterokaryons and diploids are suggested.

Recombination within the am gene of Neurospora crassa

Maps have been made showing the order of am mutant sites using (a) frequencies of am+ recombinants from crosses between am mutants and (b) the modes of distribution of the am+ recombinants among the two parental and two recombinant classes with respect to the flanking markers inos and sp.

It is possible to arrange the sites in an order such that, in almost all the crosses yielding useful numbers of am+ recombinants, the sp allele originally in coupling with the distal am mutant site occurs in the majority of the recombinants. No such consistent pattern was found with respect to the inos marker. The order obtained by reference to sp agreed with the best order deducible from recombination frequencies.

The data are consistent with the hypothesis that am+ recombinants arise by a process of gene conversion, that there is a gradient of conversion frequencies from the right (inos or distal) end of the gene to the left (sp or proximal) end, and that conversion tends to be associated (though less than 50% of the time) with crossing-over, especially on the distal side.

There is no obvious relationship between the map position of a given am mutant and the properties of the protein product of the mutant gene.

Allosteric NADP-glutamate dehydrogenase from aspergilli: purification, characterization and implications for metabolic regulation at the carbon-nitrogen interface

NADP-dependent glutamate dehydrogenase (NADP-GDH) mediates fungal ammonium assimilation through reductive synthesis of glutamate from 2-oxoglutarate. By virtue of its position at the interface of carbon and nitrogen metabolism, biosynthetic NADP-GDH is a potential candidate for metabolic control. In order to facilitate characterization, a new and effective dye-affinity method was devised to purify NADP-GDH from two aspergilli, Aspergillus niger and Aspergillus nidulans. The A. niger NADP-GDH was characterized at length and its kinetic interaction constants with glutamate (Km 34.7 mM) and ammonium (Km 1.05 mM; Ki 0.4 mM) were consistent with an anabolic role. Isophthalate, 2-methyleneglutarate and 2,4-pyridinedicarboxylate were significant inhibitors, with respective Ki values of 6.9, 9.2 and 202.0 microM. The A. niger enzyme showed allosteric properties and a sigmoid response (nH=2.5) towards 2-oxoglutarate saturation. The co-operative behaviour was a feature common to NADP-GDH from Aspergillus awamori, A. nidulans and Aspergillus oryzae. NADP-GDH may therefore be a crucial determinant in adjusting 2-oxoglutarate flux between the tricarboxylic acid cycle and glutamate biosynthesis in aspergilli.

Biochemical and molecular characterization of NADP-glutamate dehydrogenase from the ectomycorrhizal fungus Tuber borchii

• NADP-glutamate dehydrogenase (NADP-GDH) from Tuber borchii was purified and the corresponding gene was cloned in order to elucidate the physiological role of the enzyme in this ectomycorrhizal fungus. • NADP-GDH was purified using an anion-exchange column followed by affinity chromatography. The complete gene was cloned from a 30-d-old-mycelium cDNA library and characterized. • T. borchii NADP-GDH appears to be physically and kinetically similar to those from other fungi and the deduced amino acid sequence of the gdh gene showed a significant similarity to other fungal NADP-dependent GDHs. Biochemical and Northern blotting analyses carried out with mycelia grown on different nitrogen sources clearly showed that the regulation of T. borchii NADP-GDH in response to different nitrogen sources was markedly different from the responses of the NADP-GDHs of other ascomycetes. Northern blotting analyses highlighted that the gdh gene was also expressed in the symbiotic phase. • The biochemical and molecular data suggest that the fungal NADP-GDH contributes to the primary nitrogen metabolism in the ectomycorrhizal tissues.

Purification and characterization of NADP-dependent glutamate dehydrogenase from the commercial mushroom Agaricus bisporus

The nicotinamide adenine dinucleotide phosphate (NADP)-dependent glutamate dehydrogenase (NADP-GDH) of Agaricus bisporus, a key enzyme in ammonia assimilation, was purified to apparent electrophoretic homogeneity with 27% recovery of the initial activity. The molecular weight of the native enzyme was 330 kDa. The enzyme is probably a hexamer, composed of identical subunits of 48 kDa. The isoelectric point of the enzyme was found at pH 4.8. The N-terminus appeared to be blocked. The enzyme was specific for NADP(H). The Km-values were 2.1, 3.2, 0.074, 27.0, and 0.117 mM for ammonia, 2-oxoglutarate, NADPH, L-glutamate, and NADP respectively. The pH optima for the amination and deamination reactions were found to be 7.6 and 9.0, respectively. The temperature optimum was 33 degrees C. The effect of several metabolites on the enzyme's activity was tested. Pyruvate, oxaloacetate, ADP, and ATP showed some inhibitory effect. Divalent cations slightly stimulated the aminating reaction. Antibodies raised against the purified enzyme were able to precipitate NADP-GDH activity from a cell-free extract in an anticatalytic immunoprecipitation test. Analysis of a Western blot showed the antibodies to be specific for NADP-GDH.

Premeiotic disruption of duplicated and triplicated copies of the Neurospora crassa am (glutamate dehydrogenase) gene

Premeiotic inactivation of duplicated sequences (the RIP phenomenon of Selker et al.) was studied by tetrad analysis using ectopic copies of am+ (coding for NADP-specific glutamate dehydrogenase) and a missense allele am3, coding for a distinctive form of the enzyme, at the normal locus. In duplication crosses either both gene copies were inactivated or neither. Two inactivated am3 derivatives were shown to have undergone methylation and numerous base-pair changes, reflected in losses and gains of restriction sites, but without sequence rearrangement. Cutting at restriction sites within the disrupted sequences was incomplete but became almost complete following growth in the presence of 5-azacytidine. In a triplication cross in which one parent carried two unlinked ectopic gene copies together with am3 at the normal locus, premeiotic inactivation, when it occurred, tended to affect two of the three copies in any one ascus, but there were a few asci in which all three were inactivated.

A regulatory phenotype associated with the en-am 1 mutant of Neurospora crassa

We have investigated the nature of the en-am1 mutant of Neurospora crassa and have found that it affects the regulation of proline oxidase and utilisation of other nitrogen sources. This mutant is closely linked to the gln gene but not allelic with it. Data from crosses suggest that the two genes he on opposite sides of the in1 gene on linkage group VR.

Differences in patterns of complementation of the more common groups of xeroderma pigmentosum: possible implications

Xeroderma pigmentosum of groups A, C and D shows complementation differing in kinetics, dependence on the dose of wild-type alleles and dependence on protein synthesis. Such differences suggest that XP-A, -C and -D carry mutations at different loci. The product of the first of these loci (factor A) is present in significant excess in normal fibroblasts, seems to turn over rapidly and may be a dimer or higher polymer. The products of the other two loci (factors C and D) do not seem to be present in significant excess in the cytoplasm of normal fibroblasts, but factor C may accumulate abnormally in XP-D. Factors C and D turn over slowly (D more than C) and they do not move freely from the cell nucleus. Factors A and C, at least, seem to act directly and not via gene regulation.

Ageing of Neurospora crassa. I. Evidence for the free radical theory of ageing from studies of a natural-death mutant

A recessive mutant of Neurospora crassa, called natural-death, is characterized by a decreasing clonal growth potential under all nutritional conditions and the irreversible cessation of growth. The primary molecular defect of this mutant is not known. Evidence presented here, based upon measurements of the activities and thermolabilities of several enzymes, suggests that faulty protein synthesis is probably not a cause of the senescence and death of the mutant, as suggested by Lewis and Holliday (Nature, 228 (1970) 877). Three lines of evidence indicate that lipid autoxidation and associated free radical reactions contribute to the senescence and death of this mutant: (1) The relative times before the onset of senescence and death of mutant clones in the last 40% of their chronological life-span were prolonged 2 to 3-fold by either dietary antioxidants or selenite and the total life-span was increased by 40% to 80%. These compounds also alleviated the senescent morphology and enhanced biomass production; (2) Senescing clones accumulated a green fluorescent pigment in situ, but dietary antioxidant nordihydroguaiaretic acid prevented this accumulation. The fluorescent pigment exhibited the spectral properties of lipofuscin, an end product of lipid autoxidation; (3) Relative to wild type, mycelial extracts of the mutant exhibited a 2 to 4-fold excess of activities of the antioxygenic enzymes superoxide dismutase, glutathione peroxidase and glutathione reductase. We briefly review: (1) the roles of antioxygenic enzymes and antioxidants in their protection against cellular damage from lipid autoxidation and free radical reactions; and (2) the major lines of evidence which appear to support a form of the free radical theory of ageing, encompassing the interrelated processes of membrane deterioration, lipid autoxidation and deleterious free radical reactions as the major causes of cellular deterioration.

Slow conformational changes of a Neurospora glutamate dehydrogenase studied by protein fluorescence

1. The NADP-dependent glutamate dehydrogenase of Neurospora crassa undergoes slow reversible structural transitions, with half-times in the order of a few minutes, between active and inactive states. The inactive state of the enzyme, which predominates at pH values below 7.0, has an intrinsic tryptophan fluorescence 25% lower than that of the active state, which predominates at pH values above 7.6. The inactive state can be activated either by an increase in pH or by addition of activators such as succinate. 2. The kinetics of the slow transitions that follow activating and inactivating rapid changes in conditions have been monitored by measurements of protein fluorescence. The results show that the slow reversible conformational change detected by the change in fluorescence is the rate-limiting process for enzyme activation and inactivation. 3. In both directions this conformational change follows apparent first-order kinetics and the rate constant is independent of protein concentration. These kinetics and published measurements of molecular weight are indicative of an isomerization process. 4. In both directions the changes show a large energy of activation and a large positive entropy of activation, consistent with a considerable disturbance of conformation in the transition state. 5. Comparisons of the fluorescence emission spectra of the active and inactive states indicate that the difference in fluorescence is produced by quenching, possibly intramolecular, in the inactive conformation. Iodide ions cause similar quenching. 6. In some mutationally altered forms of the enzyme comparable but modified conformational changes can be followed by protein fluorescence.

Immunochemical and enzymatic studies and glutamate dehydrogenase and a related mutant protein from Neurospora crassa

Roberts, D. B. (University of Cambridge, Cambridge, England). Immunochemical and enzymatic studies on glutamate dehydrogenase and a related mutant protein from Neurospora crassa. J. Bacteriol. 91:1888-1895. 1966.-When an investigation was made of the inhibition of Neurospora glutamate dehydrogenase by bivalent and univalent antibodies, it was shown that the enzyme inhibition is not complete even with excess antibodies. The residual activity was some three times greater with bivalent antibodies, in spite of the observation that the ratio of inhibiting antibodies to catalytic sites was 2:1 for both types of antibody. Substrates did not affect the inhibition of enzyme activity, nor did antibodies affect the K(m) for either substrate. An allosteric mechanism for the inhibition of glutamate dehydrogenase by antibodies is proposed. It was also demonstrated that the mutant protein am-3 can be activated, to show glutamate dehydrogenase activity, by a number of activators. The requirement for the activation was the presence of a carboxymethyl group. The data suggest that the nonactivated protein has two combining sites for l-glutamate: the catalytic and activating sites. The wild-type enzyme has only one of these sites. Because the activating site is distinct from the catalytic site, an allosteric mechanism was postulated for activation. Inhibition of am-3 activity by antibodies is achieved either by a mechanism similar to the inhibition of wild-type activity or by the antibodies preventing the activation of the mutant protein. The am-3 protein can be activated by antibodies. Consequently, there appeared to be a relation the phenomena of enzyme inhibition and am-3 activation by antibodies; i.e., they alter the configuration of the catalytic site. This alteration was necessary for the activation of am-3, but inhibited the activity of the wild-type enzyme.

Glutamic dehydrogenase in revertants of am mutants in Neurospora

Five allelic mutants of Neurospora which lack glutamic dehydrogenase (ammutants) were induced to revert with ultra-violet. The glutamic dehydrogenase produced by the revertants was compared to that of wild-type. Several distinct classes of revertants could be distinguished by these tests. However, genetic analysis showed that all the reversions resulted from events at or near the site of the originalammutation. The spectrum of reversion types depended on the nature of theammutant employed. One mutant, which produces anamprotein believed to be altered in its active site, yielded revertants which were all indistinguishable from wild-type. Another mutant, which produces a protein with a functional active site but altered folding properties, gave at least six classes of revertants which were different from wild-type.

Slow motile mutant in Salmonella typhimurium

Enomoto, Masatoshi (National Institute of Genetics, Misima, Japan). Slow motile mutant in Salmonella typhimurium. J. Bacteriol. 90:1696-1702. 1965.-A slow motile mutant, SJ399, was isolated from a wild-type strain of Salmonella typhimurium TM2. The mutant was as motile as the wild type in broth culture at 37 C. However, on semisolid medium it produced a much narrower swarming band than TM2. The motility of this mutant was hindered by the viscosity of semisolid medium. H antigenicity and morphological characters of flagella of the mutant were the same as those of the wild type. The motility phage, chi, responded differently to SJ399 and the wild type. Plaques of SJ399 were small and cloudy, whereas on the wild type they were large and clear. The efficiency of plating on SJ399 was 0.36 as compared with 1 with the wild type. Stained preparations revealed that the mutant had about one-third the number of flagella of the wild type. The reduction of the number of flagella also was ascertained by biochemical measurement of flagellar protein which was purified after deflagellation from cells. The content of flagellin in SJ399 was about 32% of that of the wild type. Phage P22-mediated transductions from SJ399 to nonflagellated (fla(-)) and paralyzed (mot(-)) mutants showed that the mutant SJ399 complements seven fla(-) and three mot(-) strains which are representative mutants of flagellation and motility cistrons, respectively. The mutation site of SJ399 was cotransduced with both motA and B cistrons. The two point cross tests between SJ399 and mot mutants revealed that the mutation site of SJ399 is located in the motB cistron. The insertion of the genetic region containing the mutation site of SJ399 to the motB cistron is discussed in relation to intracistronic complementation.

Complementation and enzyme studies of revertants induced in an am mutant of N. crassa

A total of eighty-seven revertants were induced by ultra-violet light in anam3strain. All of these revertants appear to be the result of mutation at sites in or close to theamlocus. Fourteen of the eighty-seven revertants were partial revertants in that under some conditions of assay they possessed low glutamate dehydrogenase activity compared with the wild-type although their growth rate was similar to that of the wild-type. Enzyme extracts of thirteen of the partial revertants were assayed for glutamate dehydrogenase in various ways in order to establish qualitative distinctions between different kinds of mutant enzyme. On the basis of these tests six different groups were established, of which one contained six revertants, one three and the others one. All except one of the mutant enzyme types showed a marked activation when incubated with α-oxoglutarate plus NADPH2, and all of these had Michaelis constants for ammonium ion much higher than is found for the wild-type enzyme. The remaining group of three revertants gave, at first, no enzyme activity in any of the assay systems. Two of these (the third was not tested) were shown to produce an enzyme variety which becomes quite inactive in phosphate buffer at pH 8·0 but can be fully activated by the addition of ethylenediamine tetra-acetic acid. Forced heterocaryons between each of six partial revertants and elevenammutants were made and the resultant sixty-six heterocaryons assayed for glutamate dehydrogenase activity. The partial revertants differed among themselves in their complementation characteristics. Some complemented with none of theammutants, some witham1only, and some witham1or witham7. The complementation tests confirmed the differences established by the enzyme studies. The data presented here, together with previous work, demonstrate that ultra-violet light induced mutation in anamstrain can result in at least eight types of revertant differing from each other in respect of the glutamate dehydrogenase variety which each can produce.