Compartmental and regulatory mechanisms in the arginine pathways of Neurospora crassa and Saccharomyces cerevisiae.

Abstract

Arginine metabolism has been studied intensively in manly organisms, beginning in earnest with the discovery of the urea cycle in mammals (139). The genetic control of the arginine pathway was the earliest example of the one-gene, one-enzyme relationship in the biochemical genetics of Neiurosporca crassa (206). The elucidation of the pathway in Escherichia coli followed soon thereafter, the term "repression'' having been coined to describe the regulatory behavior of acetylornithinase (229). The discovery of carbamoyl phosphate in 1955 (131) led to many comparative studies of its metabolism (52, 130, 152). A profound knowledge of arginine metabolism in N. crassa and Sacclhiaonvces (ceev'isiae has developed in the last 25 years. The subject justifies a comparative review, because the two organisms solve similar metabolic problems in different ways. It is likely that the fundamental phenomena of compartmentation and regulation in the two organisms can be seen throughout the evolutionary tree, used variously according to the demands of particular lifestyles. The synthesis of arginine in fungi has three main components: the synthesis of ornithine, the synthesis of carbamoyl phosphate, and the conversion of these two compounds to arginine. The catabolic pathway consists of the hydrolysis of arginine to ornithine and urea, the breakdown of urea to ammonia and carbon dioxide, and the conversion of ornithine to glutamate. A theme to be pursued is how the anabolic and catabolic pathways can each proceed to the exclusion of the other, given ornithine as a common intermediate. A related theme is how carbamoyl phosphate and ornithine are confined to the arginine pathway in the face of enzymes that might divert them to other fates. Both of these matters involve the compartmentation of small molecules by intracellular membranes, which in S. (ereiisiae is supplemented by elaborate enzyme regulatory mechanisms. This review describes the enzymology, genetics, and localization of the arginine enzymes of S. cere\eisiae and N. crassa. This is followed by descriptions of vacuolar function, enzyme regulation, and the integration of relevant factors in adaptation to different environments. Metabolic maps (Fig. 1 and 2) and a gene-enzyme directory (Table 1) are given for N. crassa(and S. cerevisiae. Arginine metabolism in these organisms has been reviewed in more limited ways previously (1

Nitrogen metabolism in the ectomycorrhizal fungus Laccaria bicolor (R. Mre.) Orton

“…Under these conditions, young mycelia expanded rapidly, exhibiting distinct exponential and postexponential growth phases. The doubling time for mycelial expansion during exponential growth was about 1 d, which compares well with the cell division rates of many fast growing eukaryotic green algae (Hellebust, 1976, Ahmad & Hellebust, 1984a, 1986. We did, however, observe a growth retardation in L. bicolor, which coincided with the aggregation of hyphae into a mat-like formation after about 7 d growth in liquid media.…”

“…Our knowledge of nitrogen metabolism in fungi at present is largely confined to studies of three nonsymbiotic ascomycetes, the filamentous Emericella (Aspergillus) nidulans and Neurospora crassa, and the yeast Saccharomyces cerevisiae (Davis, 1986;Cooper, 1982;Wiame, Grenson & Arts Jr., 1985). These studies strongly suggest the presence of a transr criptional control (nitrogen catabolite control) that achieves preferential use of favoured nitrogen sources.…”

TheN‐acetylglutamate synthase/N‐acetylglutamate kinase metabolon ofSaccharomyces cerevisiaeallows co‐ordinated feedback regulation of the first two steps in arginine biosynthesis

“…The linear pattern of ornithine synthesis is found in Escherichia coli and some other bacteria and archea [1][2][3][4][5]. The cyclic pattern is more widespread among the procaryotes [6][7][8][9][10][11][12][13], and it is observed in all investigated ascomycetes, including Candida utilis [14], Saccharomyces cerevisiae [15], Neurospora crassa [2], and in Chlamydomonas algae [16]. In the fungi, ornithine synthesis proceeds entirely in the mitochondria [17,18].…”

Mitochondrial Steps of Arginine Biosynthesis are Conserved in the Hydrogenosomes of the Chytridiomycete Neocallimastix frontalis

“…Random EST sequencing uncovered an OTC transcript in a cDNA library of N. frontalis. OTC is involved in arginine biosynthesis in the mitochondria of most eukaryotes (Davis 1986), but is missing from the hydrogenosomes of the anaerobic flagellate T. vaginalis. In this organism, OTC activity is localized in the cytosol, where it is part of the ATP-generating ADH pathway (Yarlett et al 1994).…”

“…Ornithine and carbamoylphosphate are condensed by ornithine transcarbamoylase (OTC, EC 2.1.3.3) to form citrulline, which is then exported into the cytosol and converted by argininosuccinate synthase and argininosuccinate lyase to arginine. A deviation from this compartmentalization occurs in Saccharomyces cerevisiae, where CPS and OTC are located in the cytosol (Davis 1986).…”