Figure 1.

The Arabidopsis ThDP Synthesis Pathway and the Thiamin Phenotype of the th2-1 Mutant.

(A) ThDP synthesis pathway. The pyrimidine (2-methyl-4-amino-5-hydroxymethylpyrimidine diphosphate [HMP-PP]) and thiazole [4-methyl-5-(2-phosphoethyl)-thiazole (THZ-P)] moieties of thiamin are made in plastids and coupled together by TH1 to give ThMP. ThMP is then hydrolyzed to thiamin, which is converted to ThDP in the cytosol by thiamin diphosphokinases TPK1 and 2. It is not known whether ThMP is exported from plastids and dephosphorylated in the cytosol or dephosphorylated in plastids followed by thiamin export. These alternatives are shown as red and blue dashed arrows, respectively. AIR, aminoimidazole ribotide; P, phosphate group; ThMPase, ThMP phosphatase.

(B) Levels of thiamin and its phosphates in wild-type and th2-1 seedlings. Wild-type Ler-0 and th2-1 homozygote plants were cultured for 3 weeks on agar medium. Whole plants were harvested for thiamin analysis. Data are means and se for three biological replicates, each from different seedlings.