Rowland H. Davis

Carbamyl phosphate synthesis in Neurospora Crassa II. Genetics, metabolic position, and regulation of arginine-specific carbamyl phosphokinase

Abstract The sole demonstrated enzyme of carbamyl phosphate synthesis in Neurospora appears to serve only the arginine pathway. The enzyme is completely absent in arg -3 mutants, which require arginine. While pyrimidine synthesis requires carbamyl phosphate, pyrimidine synthesis is unimpaired in arg -3 mutants. This suggests the existence of a pyrimidine-specific mode of carbamyl phosphate synthesis, and gene interaction experiments in support of this notion are presented. The regulatory behavior of argine and pyrimidine enzymes in mutant strains also reinforces the hypothesis that dual modes of carbamyl phosphate synthesis prevail in this organism.

A Mutant Form of Ornithine Transcarbamylase Found in a Strain of Neurospora Carrying a Pyrimidine-Proline Suppressor Gene'

A mutation, s, of Neurospora, leads to a 98% reduction in ornithine transcarbamylase (OTC) activity without imposing an arginine requirement. It has been found that the enzyme present in the s strain is different from the normal enzyme in its affinities for ornithine and carbamyl phosphate. The affinity of the mutant enzyme for carbamyl phosphate is much greater than in the case of the wild type enzyme, and this characteristic could compensate to a large extent for the low specific activity in s strains. The data are used to indicate that the s locus is a gene which controls the structure of OTC in Neurospora. The possibility of there being two forms of this enzyme in wild type strains is discussed.

Suppressor of Pyrimidine 3 Mutants of Neurospora and Its Relation to Arginine Synthesis

The basis of the mutant phenotype of the pyr 3a strain of Neurospora appears to be the arginine sensitivity of an early step in pyrimidine synthesis. The effect of a suppressor mutation which renders pyr 3a pyrimidine-independent is to reduce arginine levels in the mycelium by its effect on ornithine transcarbamylase.

Carbamyl phosphate synthesis in Nuerospora crassa I. Preliminary characterization of arginine-specific carbamyl phosphokinase

Abstract An enzyme from Neurospora crassa which catalyzes carbamyl phosphate formation is reported. Carbamyl phosphate formation by the enzyme is hown to be dependent upon ATP, ammonia, bicarbonate, and magnesium ions. Glutamine and N- acetyl - l - glutamte , involved in carbamyl phosphate synthesis in other organisms, are not required for the reaction and do not influence it. While direct evidence for cabamate as the substrate is lacking, the enzyme is considered to bacterial carbamyl phosphokinase (ATP : carbanate phosphotransferease, EC 2.7.2.2).

Co-ordinate changes in complementation, suppression and enzyme phenotypes of a pyr-3 mutant of Neurospora crassa

Co-ordinate changes in complementation, suppression and enzyme phenotypes of a pyr-3 mutant of Neurospora crassa

Acetylglutamate kinase: A feedback-sensitive enzyme of arginine biosynthesis in neurospora

Abstract A radioactive assay was developed for the arginine-synthetic enzyme, acetylglutamate kinase (EC 2.7.2.8). Activity of the enzyme was demonstrated in crude extracts of Neurospora mycelium. Precipitation with ammonium sulfate, resulting in separation of the enzyme from an inhibitor, was initially required to detect activity. Most preparations are only partially sensitive to arginine, with maximal inhibition achieved at an effector concentration of 0.5 mM. The enzyme is activated about 10% by 1 mM lysine or citrulline, while 1 mM ornithine stimulates activity by 75%.