EVIDENCE FOR THE THIAMINE BIOSYNTHETIC-PATHWAY IN HIGHER-PLANT PLASTIDS AND ITS DEVELOPMENTAL REGULATION

Thiamine or vitamin B-1, is an essential constituent of all cells sinc e it is a cofactor for two enzyme complexes involved in the citric aci d cycle, pyruvate dehydrogenase and cc-ketoglutarate dehydrogenase. Th iamine is synthesized by plants, but it is a dietary requirement for h umans and other animals. The biosynthetic pathway for thiamine in plan ts has not been well characterized and none of the enzymes involved ha ve been isolated. Here we report the cloning and characterization of t wo cDNAs representing members of the maize thi1 gene family encoding a n enzyme of the thiamine biosynthetic pathway. This assignment was mad e based on sequence homology to a yeast thiamine biosynthetic gene and by functional complementation of a yeast strain in which the endogeno us gene was inactivated. Using immunoblot analysis, the thi1 gene prod uct was found to be located in a plastid membrane fraction. RNA gel bl ot analysis of various tissues and developmental stages indicated thi1 expression was differentially regulated in a manner consistent with w hat is known about thiamine synthesis in plants. This is the first rep ort of cDNAs encoding proteins involved in thiamine biosynthesis for a ny plant species.