Regulation of thymidine metabolism in Neurospora crassa

Abstract

The utilization of thymidine by Neurospora crassa is initiated by the pyrimidine deoxyribonucleoside 2'-hydroxylase reaction and the consequent formation of thymine and ribose. Thymine must then be oxidatively demethylated by the thymine 7-hydroxylase and uracil-5-carboxylic acid decarboxylase reactions. This article shows that the 2'-hydroxylase reaction can be regulated differently than the oxidative demethylation process and suggests that the 2'-hydroxylase has, in addition to the role of salvaging the pyrimidine ring, the role of providing ribose not only for the utilization of the demethylated pyrimidine but also for other metabolic processes. One way that this difference in regulation was observed was with the uc-1 mutation developed by Williams and Mitchell. The present communication shows that this mutation increases the activities of the 7-hydroxylase and the decarboxylase but has no comparable effect on the 2'hydroxylase. Qualitatively similar effects on these enzymes were brought about by growth of wild-type Neurospora in media lacking ammonium ion, such as the Westergaard-Mitchell medium. The 2'-hydroxylase and 7-hydroxylase are also differently affected by the carbon dioxide content of the atmosphere above the growing culture and the growth temperature. Studies with inhibitors indicated that the carbon dioxide effect is dependent on protein synthesis. Recent studies (34) have indicated that in order for Neurospora crassa to utilize pyrimidine deoxyribonucleosides they must be initially metabolized as depicted in Fig. 1. This report describes several factors that may be pertinent to the regulation of the pathway. The first indication of one ofthese factors came from attempts to facilitate purification of thymine 7hydroxylase by growing liquid cultures under forced aeration. Although the yield of mycelia was increased over that obtained from shallow liquid cultures, the 7-hydroxylase activity was markedly decreased and, furthermore, the incorporation of thymidine into ribonucleic acid was inhibited (22). However, larger nonaerated cultures with considerable 7-hydroxylase activity could be obtained if the Neurospora were grown with stirring in a sealed carboy halffilled with medium (22). Since the atmosphere inside of the carboy became anaerobic during growth, it was suspected that an increase in the 7-hydroxylase activity might, somehow, be initiated by this change in the gas composition.Another factor that may play a regulatory role is the ammonium ion content of the growth medium. The Westergaard-Mitchell medium permits the efficient utilization of intermediates in the pathway shown in Fig. 1 (39). The data in the present article indicate that mycelia grown in this medium contain higher activities

Chromosomal loci of Neurospora crassa

“…Uracil salvage pathway, by which many exogenously supplied pyrimidines are cycled via uracil to uridine 5'-phosphate and, thus, into the latter part of the pyrimidine biosynthetic pathway. The figure shows the sites of action of the salvage pathway uc genes, the ud and uc-5 genes, which control uptake of exogenous uridine and uracil, and udk, which controls uridine kinase (126,420,616,977,1141). For endogenous pyrimidine synthesis, see Fig.…”

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

“…Another established pathway of pyrimidine catabolism involves the oxidation of the methyl group of thymine but does not degrade the pyrimidine ring (11,13). This oxidative pathway has been detected in Neurospora (11), in Rhodotorula (29,33,34), and in mammals (10).…”