THI1, A THIAMINE BIOSYNTHETIC GENE IN ARABIDOPSIS-THALIANA, COMPLEMENTS BACTERIAL DEFECTS IN DNA-REPAIR

An Arabidopsis thaliana cDNA was isolated by complementation of the Es cherichia coli mutant strain BW535 (xth, nfo, nth), which is defective in DNA base excision repair pathways. This cDNA partially complements the methyl methane sulfonate (MMS) sensitive phenotype of BW535. It a lso partially corrects the UV-sensitive phenotype of E. coli AB1886 (u vrA) and restores its ability to reactivate UV-irradiated lambda phage . It has an insert of ca. 1.3 kb with an open reading frame of 1047 bp (predicting a protein with a molecular mass of 36 kDa). This cDNA pre sents a high homology to a stress related gene from two species of Fus arium (sti35) and to genes whose products participate in the thiamine biosynthesis pathway, THI4, from Saccharomyces cerevisiae and nmt2 fro m Schizosaccharomyces pombe. The Arabidopsis predicted polypeptide has homology to several protein motifs: amino-terminal chloroplast transi t peptide, dinucleotide binding site, DNA binding and bacterial DNA po lymerases. The auxotrophy for thiamine in the yeast thi4::URA3 disrupt ion strain is complemented by the Arabidopsis gene. Thus, the cloned g ene, named thi1, is likely to function in the biosynthesis of thiamine in plants. The data presented in this work indicate that thi1 may als o be involved in DNA damage tolerance in plant cells.