Pyrimidine biosynthesis in Aspergillus nidulans. Isolation and characterisation of mutants resistant to fluoropyrimidines.

Mutants resistant to 5-fluorouracil, 5-fluorouridine and 5-fluorodeoxyuridine have been selected in Aspergillus nidulans. Growth tests combined with genetic analysis showed that mutations conferring resistance to fluoropyrimidines could occur in at least seven genes. Three of these fulE, fulF and furA were concerned with either the uptake of pyrimidines or their conversion to uridine monophosphate. The other four genes did not affect these functions. Mutations in fulA probably confer resistance by lowering ornithine transcarbamoylase, thereby making the normally arginine-specific carbamoyl phosphate pool available for increased uracil synthesis. Mutations in fulD may make the arginine-specific carbamoyl phosphate synthetase insensitive to inhibition or repression by arginine, and so lead to increased carbamoyl phosphate pool sizes, and increased uracil synthesis. Both fulA and fulD mutants suppress pyrA mutants which lack the uracil-specific carbamoyl phosphate synthetase. Mutations in fulB and fulC do not suppress pyrA, and so may act more directly to increase uracil synthesis. The synthesis of aspartate carbamoyl transferase in fulB7 strains is not repressed by uracil. fulC mutants are closely linked to the pyrA, B, C, N region which codes for the first two enzymes of pyrimidine biosynthesis, and may result in these enzymes being less sensitive to inhibition by uracil.