Isolation and characterization of thymidylate synthetase mutants of Xanthomonas maltophilia.

Thymidylate synthetase mutants of Xanthomonas maltophilia ATCC 13270 were isolated on a solid minimal medium containing 50 mg/l thymidine and a high concentration of trimethoprim (500 mg/l). It was found that a high concentration of trimethoprim was required to prevent background growth of the wild-type strain. The isolated mutants could grow on thymidine or dTMP at a concentration of 50 mg/l while they were unable to grow on 1000 mg/l thymine or 50 mg/l deoxyuridine. Thymidylate synthetase activity was assayed in the wild-type cells and in the mutant cells but only the wild-type cells contained measurable enzyme activity.