Thiamin biosynthesis in prokaryotes

Twelve genes involved in thiamin biosynthesis in prokaryotes have been iden tified and overexpressed. Of these, six are required for the thiazole biosy nthesis (thiFSGH, thiI, and dxs), one is involved in the pyrimidine biosynt hesis (thiC), one is required for the linking of the thiazole and the pyrim idine (thiE), and four are kinase genes (thiD, thiM, thiL, and pdxK). The s pecific reactions catalyzed by ThiEF, Dxs, ThiDM, ThiL, and PdxK have been reconstituted in vitro and ThiS thiocarboxylate has been identified as the sulfur source. The X-ray structures of thiamin phosphate synthase and 5-hyd roxyethyl-3-methylthiazole kinase have been completed. The genes coding fur the thiamin transport system (thiBPQ) have also been identified. Remaining problems include the cloning and characterization of thiK (thiamin kinase) and the gene(s) involved in the regulation of thiamin biosynthesis. The sp ecific reactions catalyzed by ThiC (pyrimidine formation), and ThiGH and Th iI (thiazole formation) have not yet been identified.