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Metabolism and Enzymology

A th-1 Mutant of Arabidopsis thaliana Is Defective for a Thiamin-Phosphate-Synthesizing Enzyme: Thiamin Phosphate Pyrophosphorylase ¹

Molecular Genetics Research Laboratory, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan, Osaka Prefectural Institute of Public Health, Nakamichi, Higashinari-ku, Osaka 537, Japan

We have examined the activity of the thiamin phosphate pyrophosphorylase in Arabidopsis thaliana wild type and in a mutant (th-1) which requires exogenous thiamin for growth. Mutant and wild-type plants grown in 1 x 10^–7 molar thiamin were used for the examination of the production of thiamin and thiamin monophosphate (TMP) using 4-methyl-5-hydroxyethylthiazole phosphate and 2-methyl-4-amino-5-hydroxymethylpyrimidine pyrophosphate as substrates. While the wild-type strain formed both thiamin and TMP, the th-1 mutant did not. When TMP was added to the extracts, the th-1 mutant, as well as wild type, produced thiamin. Accordingly, it was concluded that the th-1 mutant was defective in the activity of TMP pyrophosphorylase. Some of the characteristics of the enzyme from the wild-type plant were examined. The optimum temperature for the reaction is 45°C, and the K_m values for the substrates are 2.7 x 10^–6 molar for 4-methyl-5-hydroxyethylthiazole phosphate and 1.8 x 10^–6 molar for 2-methyl-4-amino-5-hydroxymethylpyrimidine pyrophosphate.

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