Conditional Antifolate Resistance in Bacillus subtilis thyA

Resistance to antifolates in Bacillus subtilisstrains results from the presence of an antifolate resistance mutation (afo). Strains which are thyA+afoare unconditionally resistant to antifolates. The conditional resistance of thyA afostrains is hypothesized to be due to the thyB+gene product (thymidylate synthetase B) having a high Kmfor 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. subtilisthymine-requiring mutants are afo. Analysis of dihydrofolate reductase from afocells did not reveal an obvious mechanism for antifolate resistance in those cells.