The Origin and Significance of the THI5 Gene Family in Saccharomyces cerevisiae

The Saccharomyces cerevisiae THI5 gene family consists of four highly conserved members; THIS, THI11, THI12 and THI13. Each member is located within the subtelomeric region of a different chromosome and they share these regions with members of other gene families. A detailed analysis of the genome environment of each THI5 family member has shown all four to exist with their neighbouring genes in a highly conserved arrangement. A survey of THI5 copy number among a selection of hemiascomycetes suggests that the existence of THIS as a gene family is exclusive to those yeasts of the Saccharomyces sensu stricto complex. Furthermore, the association and organisation with respect to neighbouring members of other subtelomeric gene families appears to be conserved between S. cerevisiae and other sensu stricto yeasts. This conservation of local gene order does not appear to extend to yeasts outside of this subgroup. S. cerevisiae strains have been constructed which contain deletions of THI5, THI11, THI12 and THI13 in all possible combinations to yield the single, double, triple and quadruple mutants. Phenotypic analysis of the quadruple deletion mutant has found it to exhibit thiamin (vitamin B1) auxotrophy on medium containing glucose as the carbon source. This auxotrophy can be remedied by the exogenous addition of thiamin or one of its precursors hydroxymethyl-pyrimidine (HMP). Analysis of other mutant strains has shown that the THI5 gene family members are functionally redundant, with each encoded isozyme having an apparent equal role in HMP formation from pyridoxine (vitamin B6). Physiological studies of these mutant strains have examined the regulation of each of the four genes by thiamin and its precursors, as well as investigating a recently proposed anaerobic HMP biosynthetic pathway.