Conditional antifolate resistance in Bacillus subtilis thyA.

Resistance to antifolates in Bacillus subtilis strains results from the presence of an antifolate resistance mutation (afo). Strains which are thyA(+)afo are unconditionally resistant to antifolates. The conditional resistance of thyA afo strains is hypothesized to be due to the thyB(+) gene product (thymidylate synthetase B) having a high K(m) for the folate substrate, thus leading to thymineless death in the presence of antifolates. An alternative model for conditional antifolate resistance was shown to be incorrect by analysis of folate metabolism in methotrexate-treated cells. Genetic analysis and studies of the response of afo(+) cells to methotrexate suggested that most, if not all, B. subtilis thymine-requiring mutants are afo. Analysis of dihydrofolate reductase from afo cells did not reveal an obvious mechanism for antifolate resistance in those cells.