Utilization of exogenous pyrimidines as a source of nitrogen by cells of the yeast Rhodotorula glutinis

Abstract

Uptake and intracellular transformation of pyrimidines supplying cells of the yeast Rhodotorula glutinis with nitrogen have been studied. The amine nitrogen of cytosine was found to be the easiest to utilize. The presence in the medium of inorganic ammonia along with cytosine had a slight effect on cytosine deaminase (EC 3.5.4.1) activity. The uracil produced entered into the nutrient medium with no fission break of the pyrimidine ring. In the absence of any other source of nitrogen, the cells of the yeast R. glutinis utilized nitrogen of the pyrimidine ring of oxypyrimidines. Catabolism of uracil followed the reductive pattern, with release of carbon dioxide; this was accompanied by synthesis of the key enzyme of pyrimidine catabolism, dihydrouracil dehydrogenase (EC 1.3.1.1), whose activity rose 10-fold. With thymine as the sole source of nitrogen, the lagphase growth of the yeast cells was maximum. Catabolism of the pyrimidine ring of thymine was possibly preceded by its transformation into uracil. With no source of nitrogen easily utilized, the uridine 5'-monophosphate content in the generally acid-soluble pool rose. Our discussion of the regulation of catabolism of exogenous pyrimidine bases by the yeast R. glutinis takes into account the fact that transformations of pyrimidine bases are determined by how easily the cells can use a particular base as a source of nitrogen.

A Salmonella typhimurium Strain Defective in Uracil Catabolism and -Alanine Synthesis

“…1) consists of three enzymic steps which culminate in the release of p-alanine, carbon dioxide and ammonia (Vogels & van der Drift, 1976). This reductive catabolic pathway of uracil degradation appears to occur quite often in both prokaryotic (Campbell, 1957;Kramer & Kaltwasser, 1969;Ban et al, 1972;Hilton et al, 1975) and eukaryotic organisms (Woodward et al, 1957;Milstein & Bekker, 1976;Wasternack et al, 1979;Kaspari, 1981). The decarboxylation of aspartic acid by the enzyme aspartate-ldecarboxylase (EC 4.1 .…”

Urea: obligate intermediate of pyrimidine-ring catabolism in Rhodosporidium toruloides

“…Supplementation with dihydrouracil. Although dihydrouracil is able to serve as a sole source of nitrogen (Table 1), we were not able to confinn that Rhodosporidium converts uracil to dihydrouracil and N-carbamyl-fi-alanine as has been reported (20,29). In an experiment designed to follow the fate of dihydrouracil, cells were grown for 6 days in minimal medium supplemented with 500 ,jg of dihydrouracil per ml.…”

“…In this paper we report the finding that urea is an obligate intermediate in the catabolism of pyrimidines in Rhodosporidium, the designation given to interfertile members of Rhodotorula, etc., by Banno (1). This finding is unanticipated in previous papers concerning Rhodotorula in which elements of the reductive pathway have been shown (20,29). Thus Rhodosporidium and mammals may have a similar array of enzymes involved with pyrimidine catabolism.…”

“…The third pathway involves the reduction of the pyrimidine ring so that uracil is converted to dihydrouracil, N-carbamyl-,8-alanine, and ,8-alanine. This reductive pathway has been ascribed to a wide variety of organisms including the procaryotes Clostridium (4,15) and Hydrogenomonas (16) and a broad spectrum of eucaryotes including mammals (12), higher plants (28), Euglena (31), Neurospora (35), Saccharomyces and Torulopis (23), and Rhodotorula (20,29). In the cases of Neurospora and Rhodotorula, a functional demonstration of the complete pathway is lacking.…”