Vitamin B1 biosynthesis in plants requires the essential iron–sulfur cluster protein, THIC

Abstract

Vitamin B1 (thiamin) is an essential compound in all organisms acting as a cofactor in key metabolic reactions and has furthermore been implicated in responses to DNA damage and pathogen attack in plants. Despite the fact that it was discovered almost a century ago and deficiency is a widespread health problem, much remains to be deciphered about its biosynthesis. The vitamin is composed of a thiazole and pyrimidine heterocycle, which can be synthesized by prokaryotes, fungi, and plants. Plants are the major source of the vitamin in the human diet, yet little is known about the biosynthesis of the compound therein. In particular, it has never been verified whether the pyrimidine heterocycle is derived from purine biosynthesis through the action of the THIC protein as in bacteria, rather than vitamin B6 and histidine as demonstrated for fungi. Here, we identify a homolog of THIC in Arabidopsis and demonstrate its essentiality not only for vitamin B1 biosynthesis, but also plant viability. This step takes place in the chloroplast and appears to be regulated at several levels, including through the presence of a riboswitch in the 3-untranslated region of THIC. Strong evidence is provided for the involvement of an iron-sulfur cluster in the remarkable chemical rearrangement reaction catalyzed by the THIC protein for which there is no chemical precedent. The results suggest that vitamin B1 biosynthesis in plants is in fact more similar to prokaryotic counterparts and that the THIC protein is likely to be the key regulatory protein in the pathway.metabolites ͉ plant viability ͉ riboswitch ͉ thiamin ͉ Arabidopsis

Overexpression of Plastid Transketolase in Tobacco Results in a Thiamine Auxotrophic Phenotype

“…Tobacco mutants in thiamine biosynthesis exhibited a 50% reduction in chlorophyll pigments that was also fully reversible by the addition of exogenous thiamine (McHale et al, 1988). A similar result was also obtained in the Arabidopsis ThiC insertion mutants that were blocked in the biosynthesis of the pyrimidine moiety of the thiamine molecule, producing albino plants that exhibited slow growth, even though starch and sucrose levels accumulated up to 40 and 50% more, respectively, than wild-type plants (Raschke et al, 2007;Kong et al, 2008).…”

AtTHIC, a gene involved in thiamine biosynthesis in Arabidopsis thaliana

“…AtTHIC was expressed strongly in leaves, flowers and siliques, but only with a trace expression in roots (Figure 9A). Raschke et al [34] recently reported that the Considering that AtTHIC is involved in thiamine biosynthesis, we analysed the effect of extrinsic thiamine supplementation on AtTHIC expression. One-week old seedlings were transferred to MS plates supplied with different thiamine concentrations (0, 0.5, 1.0, 10, 100 and 1 000 mg/L) and were grown for 1 day.…”

“…The arrangement of the two functional domains in one protein results in the coordinated synthesis of the two enzymes for thiamine biosynthesis. More recently, Raschke et al [34] described that Arabidopsis THIC is an iron-sulfur cluster protein and is involved in thiamine biosynthesis.…”

Orchestration of Thiamin Biosynthesis and Central Metabolism by Combined Action of the Thiamin Pyrophosphate Riboswitch and the Circadian Clock in Arabidopsis

“…In Arabidopsis thaliana, three enzymes participate together in the synthesize of thiamin monophosphate (TMP), namely, TH1 (Ajjawi et al, 2007a), THI1 (Belanger et al, 1995;Machado et al, 1996), and the TPP riboswitch-regulated THIC (Bocobza et al, 2007;Raschke et al, 2007;Wachter et al, 2007;Kong et al, 2008). Unlike in prokaryotes where TMP can be directly converted into TPP (Begley et al, 1999), in Arabidopsis, TMP is subsequently dephosphorylated into thiamin (Komeda et al, 1988), which is then pyrophosphorylated into TPP by the thiamin pyrophosphokinases (TPKs), namely, TPK1 and TPK2 (Ajjawi et al, 2007b).…”