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

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 degrees 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.