title: 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 micrograms/ml) of thymidine in addition to a requirement for N-5-formyl tetrahydrofolic acid (folinate). Even at high concentrations (up to 500 micrograms/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 10(3)-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 10(2)-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.
mesh_terms: D000630:Aminopterin / Q000494:pharmacology; D005492:Folic Acid / Q000378:metabolism; D002955:Leucovorin / Q000502:physiology; D008727:Methotrexate / Q000494:pharmacology; D008780:Methyltransferases / Q000378:metabolism*; D009154:Mutation; D010376:Pediococcus / Q000201:enzymology / Q000235:genetics / Q000502:physiology*; D013762:Tetrahydrofolate Dehydrogenase / Q000378:metabolism*; D013936:Thymidine / Q000494:pharmacology / Q000502:physiology*; D013940:Thymidylate Synthase / Q000378:metabolism*
chemical_list: D005492:Folic Acid; D013762:Tetrahydrofolate Dehydrogenase; D008780:Methyltransferases; D013940:Thymidylate Synthase; D000630:Aminopterin; D002955:Leucovorin; D013936:Thymidine; D008727:Methotrexate