The THI5 gene family of Saccharomyces cerevisiae: distribution of homologues among the hemiascomycetes and functional redundancy in the aerobic biosynthesis of thiamin from pyridoxine

Abstract

TheTHI5gene family ofSaccharomyces cerevisiaecomprises four highly conserved members namedTHI5(YFL058w),THI11(YJR156c),THI12(YNL332w) andTHI13(YDL244w). Each gene copy is located within the subtelomeric region of a different chromosome and all are homologues of theSchizosaccharomyces pombenmt1gene which is thought to function in the biosynthesis of hydroxymethylpyrimidine (HMP), a precursor of vitamin B1, thiamin. A comprehensive phylogenetic study has shown that the existence ofTHI5as a gene family is exclusive to those yeasts of theSaccharomyces sensustrictosubgroup. To determine the function and redundancy of each of theS. cerevisiaehomologues, all combinations of the single, double, triple and quadruple deletion mutants were constructed using a PCR-mediated gene-disruption strategy. Phenotypic analyses of these mutant strains have shown the four genes to be functionally redundant in terms of HMP formation for thiamin biosynthesis; each promotes synthesis of HMP from the pyridoxine (vitamin B6) biosynthetic pathway. Furthermore, growth studies with the quadruple mutant strain support a previous proposal of an alternative HMP biosynthetic pathway that operates in yeast under anaerobic growth conditions. Comparative analysis of mRNA levels has revealed subtle differences in the regulation of the four genes, suggesting that they respond differently to nutrient limitation.