Summary: A mutant of highly resistant to 2-amino-4-methyl-5-β-hydroxyethylthiazole (2-aminohydroxyethylthiazole), an antimetabolite of 4-methyl-5-β-hydroxyethylthiazole (hydroxyethylthiazole), has been isolated. Its resistance to 2-aminohydroxyethylthiazole was about 10 times that of the sensitive parent strain. The amount of thiamin synthesized in the cells of the resistant strain grown in minimal medium was less than half of that of the sensitive strain. The ability to synthesize thiamin from 2-methyl-4-amino-5-hydroxymethylpyrimidine (hydroxymethylpyrimidine) and hydroxyethylthiazole in the resistant strain was low compared with that of the sensitive strain. These results were found to be due to a deficiency of hydroxyethylthiazole kinase in the resistant strain: in sonic extracts of cells the enzyme activity was only 0·67% of that of the sensitive strain. Although the cells of the sensitive strain could accumulate exogenous hydroxyethylthiazole in the form of hydroxyethylthiazole monophosphate, no significant uptake of hydroxyethylthiazole by the cells of the resistant strain was observed. The possibilities that 2-aminohydroxyethylthiazole monophosphate may be the actual inhibitor of the growth of , and that hydroxyethylthiazole may not be involved in the pathway of synthesis of thiamin via hydroxyethylthiazole monophosphate, are discussed.


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