• YAKHAK HOEJI
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• v.36 no.4
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• pp.345-349
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• 1992

Thymidylate synthase activity from extracts of various methotrexate-resistant strains was measured by spectrophotometric assay. Methotrexate-resistant strains of Lactobacillus, Pseudomonas sp., Micrococcus sp. HS-1, Klebsillela pneumonae, Cellulomonas fimi and Serratia marcescens elevated thymidylate synthase levels, especially, Pseudomonas sp. KL-9 resistant to $10^{-9}M$ methotrexate have a 156-fold increase in thymidylate synthase, which suggests that Pseudomonas sp. is a convenient source of thymidylate synthase. Several methotrexate strains of yeast were tested, however, their enzyme activity was generally lower than that of bacteria tested.