Genetic evidence on the organization and action of the qa-1 gene product: a protein regulating the induction of three enzymes in quinate catabolism in Neurospora crassa

The first three reactions in the catabolism of qainic acid in Neurospora crassa are under the genetic control of the qa gene cluster. This cluster consists of three structural genes encoding three inducible enzymes plus a regulatory gene (qa-1+) whose diffusible product apparently acts in a positive fashion to initiate coordinate synthesis of the three enzymes when an appropriate inducer is present. Genetic and biochemical evidence for both complementing and temperature-sensitive qa-1 alleles indicates that the product of the qa-1+ gene is an oligomeric (multimeric) protein. On the basis of cis-trans tests of appropriate double mutants (plus genetic mapping data for temperature-sensitive mutants), at least certain constitutive mutants (which produce all three qa enzymes in the absence of an inducer) are mutants in the regulatory gene and not in controlling elements such as initiators. The detection of stable (non-revertible) qa-1 intralocus deletion (multisite)mutants provides additional evidence for positive regulation in the qa system. Extensive genetic recombination data provide evidence that the two types of qa-1 mutants--slow-complementing (qa-1-s) and fast-complementing (qa-1-f)--map in discrete, non-overlapping segments of the qa-1 locus. These two distinct types of mutants are hypothesized to produce altered regulatory protein molecules that fail to interact either with a DNA initiator site (qa-1-s types) or with an inducer (qa-1-f types). The striking similarities between the qa system in this lower eukaryote and certain prokaryote operon systems are discussed.

Subunit structure and some properties of pyruvate kinase of Neurospora

Pyruvate kinase isolated from Neurospora and purified to homogeneity has been shown to be a tetramer of molecular weight around 242 000 by gel filtration studies and 239 000 daltons by sedimentation equilibrium measurements. The monomer produced by treatment with guanidine hydrochloride is found to be 51 000-52 000 daltons by sedimentation equilibrium studies; a molecular weight of 62 000 was determined for the monomer generated by SDS treatment by electrophoresis in SDS-polyacrylamide gels. The enzyme has an isoelectric point of 6.35-6.41; Substrate saturation kinetics of PEP show a variable extent of cooperativity depending upon the buffer ions employed in the assay. ADP is the most effective phosphoryl group acceptor, GDP and IDP being poor substitutes. A divalent cation, Mg-2+, is required for activity. At low concentrations, Ca-2+ acts as an activator of pyruvate kinase but it is inhibitory at high concentrations. Fructose 1,6-diphosphate is the most potent allosteric activator, fructose 6-phosphate being next in order of effectiveness. Valine is a powerful inhibitor. Phenylalanine, tyrosine, and tryptophan are without any effect individually, but their simultaneous presence results in a considerable activation. Alanine does not affect this enzyme appreciably.

Enzymes of deoxythymidine triphosphate biosynthesis in Neurospora crassa mitochondria

Intact mitochondria of Neurospora crassa incorporate deoxythymidine 5'-monophosphate (dTMP) into deoxyribonucleic acid but not the label from (methyl-3H) deoxythymidine. Mitochondrial homogenates contain deoxythymidylate kinase (EC 2.7.4.9), deoxycytidylate aminohydrolase (dCMP deaminase) (EC 3.5.4.12), and thymidylate synthetase (EC 2.1.1b), but not thymidine kinase (EC 2.7.1.21) activity. dTMP kinase is loosely bound to the mitochondrial membrane and is solubilized by 0.4 M KCl in mitochondrial homogenates, the dCMP aminohydrolase deaminase) is bound to the inner membrane and is not solubilized by 0.4 M KCl. dTMP synthetase activity is found in the 2,000 times g particulate fractions by homogenization of mitochondria in 0.4 M KCl. The dCMP deaminase activity found in the particulate fraction of the inner membrane is efficiently regulated by the products of the pathway: deoxycytidine 5'-triphosphate activates whereas deoxythymidine 5'-triphosphate inhibits, as found for the soluble enzyme from other sources. These data indicate that mitochondria of N. crassa contain specific enzymes for the biosynthesis of deoxythymidine triphosphate.

Solubilization of valine-sensitive acetohydroxy acid synthetase from Neurospora mitochondria

Acetohydroxy acid synthetase [EC 4.1.3.18] of Neurospora crassa has been solubilized from a mitochondrial pellet fraction by sonic treatment in 1.0 M potassium phosphate buffer at pH 7.5 containing 10 mM MgSO4 and centrifugation at 180,000 X g for 20 min. The soluble enzyme thus obtained has a high specific activity and a high sensitivity to valine comparable to the original mitochondrial pellet suspension when the activity was determined in high concentrations of potassium phosphate.

[Repression of urease biosynthesis in Neurospora crassa by ammonium ions]

The regulation of the synthesis of the enzyme urease (urea amido hydrolase E.C. 3.5.1.5.) in Neurospora crassa was investigated. The biosynthesis of urease is repressed by ammonium ions. Under ammonium excess conditions the specific activity of urease decreases from 0.980 to 0.180 mumoles NH3/min/mg protein. By addition of cycloheximide it was shown that ammonia influences the synthesis of this enzyme. Enzyme induction by the substrate could be excluded. Even under the conditions of highest repression a specific activity of urease of 0.180 mumoles NH3/min/mg protein was measured. Possible causes of this constitutive enzyme level are discussed.

Enzymes of Neurospora crassa which attack UV-irradiated DNA

Two excision-deficient mutants of Neurospora crassa contain normal levels of two enzymes, a single-strand-specific exonuclease and a single-strand specific endonuclease, which attack UV-irradiated DNA. Mutants of N. crassa have been obtained in which the activity of the latter enzyme as well as an activity with native DNA are simultaneously affected. These activities are also apparently low in excision-proficient uvs3 strain of N. crassa which has many of the characteristics of the recA mutants of E. coli.

Rhythms of enzyme activity associated with circadian conidiation in Neurospora crassa

The mycelial growth front of the band strain of Neurospora grown on a solid surface exhibits a circadian rhythm of conidiation. Enzyme assays on extracts from that mycelium have shown that the activities of 6 of 13 enzymes (nicotinamide adenine dinucleotide nucleosidase, isocitrate lyase, citrate synthase, glyceraldehydephosphate dehydrogenase, phosphogluconate dehydrogenase, and glucose-6-phosphate dehydrogenase) and soluble-protein content oscillate with the visible morphological change. The rhythmic enzymes associated with the Krebs and glyoxylate cycles are more active during conidiogenesis, whereas the activities of the rhythmic enzymes of glycolysis and the hexose monophosphate shunt are reduced during that phase. The absence of enzyme oscillations in wild-type and fluffy strains which do not form conidia under the conditions employed suggests that the enzyme fluctuations are associated with conidiogenesis itself. Oscillations of enzyme activity as a function of time are restricted to the growth front. A permanent record of rhythmicity associated with conidial and nonconidial regions does, however, exist in the mycelial mat behind the growth front. The activities of three enzymes (nicotinamide adenine dinucleotide nucleosidase, glucose-6-phosphate dehydrogenase, and phosphogluconate dehydrogenase) are not directly influenced by CO(2) concentration, but are correlated with the prescence or absence of conidiation which is controlled by CO(2) concentration. In contrast, citrate synthase and malate dehydrogenase activities are correlated with changes in CO(2) concentration.