Tomato LeTHIC is an Fe-requiring HMP-P synthase involved in thiamine synthesis and regulated by multiple factors.

Thiamine is a key primary metabolite which is necessary for the viability of all organisms. It is a dietary requirement for mammals because only prokaryotes, fungi and plants are thiamine prototrophs. In contrast to the well documented biosynthetic mechanism in bacteria, much remains to be deciphered in plants. In this work, a tomato thiamine-auxotrophic (thiamineless, tl) mutant was characterized. The tl mutant occurs due to inactivation of LeTHIC transcription as a result of insertion of a large unknown DNA fragment in its 5'-untranslated region. Expression of wild-type LeTHIC in tl plants was able to complement the mutant to wild type. LeTHIC possessed the same function as E.cTHIC [an Escherichia coli 4-amino-5-hydroxymethyl-2-methylpyrimidine phosphate (HMP-P) synthase involved in synthesis of the pyrimidine moiety of thiamine] because expression of LeTHIC rescued THIC-deficient strains of E. coli under culture conditions without thiamine supplementation, suggesting that plants employ a bacteria-like route of pyrimidine moiety synthesis. LeTHIC is an Fe-S cluster protein localized in chloroplasts, and Fe is required for maintenance of its enzyme activity because Fe deficiency resulted in a significant reduction of thiamine content in tomato leaves. Further, we also showed that the expression of LeTHIC is tightly regulated at the transcriptional and post-transcriptional level by multiple factors, such as light, Fe status and thiamine pyrophosphate (TPP)-riboswitch. The results clearly demonstrated that a feedback regulation mechanism is involved in synthesis of the pyrimidine moiety for controlling thiamine synthesis in tomato. Our results provide a new insight into understanding the molecular mechanism of thiamine biosynthesis in plants.