Physiological and Enzymatic Properties of a Thymidine-Requiring Pediococcus cerevisiae Mutant

Abstract

We describe the isolation and characterization of a Pediococcus cerevisiae thymidine-requiring mutant and its thymidine-independent revertant. The mutant strain lacked thymidylate synthetase activity and had an absolute requirement for low concentrations (2 ,ug/ml) of thymidine in addition to a requirement for N-5formyl tetrahydrofolic acid (folinate). Even at high concentrations (up to 500 ,ug/ ml), thymine could not replace thymidine. In contrast to its wild-type parent, which grows only on folinate, the thymidine-requiring mutant (Thy-Fol+) was able to take up and grow on picogram quantities of unreduced folic acid. When both strains were grown on folinate, the Thy-Fol+ strain was at least 103-fold more resistant to the folic acid analogs aminopterin and methotrexate than the wild-type strain. On the other hand, when grown on folic acid, the Thy-Fol+ strain was as sensitive to the folic acid analogs as the Thy' Fol+ strain and was 102-fold more sensitive than the wild-type strain grown on folinate. The thymidine-independent revertant (Thy' Fol+) regained the wild-type level of thymidylate synthetase activity, but maintained the ability to take up and grow on unreduced folic acid like its Thy-Fol+ parent.