Abstract

The ascomycetes S. cerevisiae and C. albicans metabolize the pentose sugar xylose differently. S. cerevisiae cannot grow on xylose as the sole carbon source while C. albicans can. In the xylose-metabolic pathway, xylose is reduced to xylitol by a xylose reductase (GRE3), then oxidized to xylulose by a xylose dehydrogenase (ScSOR1, CaXYL2). These two enzymes are essential for the xylose metabolic pathway. Although S. cerevisiae does not grow in xylose medium, it has been shown that the enzymes have enzymatic potential when used to complement various C. albicans knockouts. C. albicans was used as a model to study the function of various pairings of the reductase and dehydrogenase from the two ascomycetes. Transcription profiling confirmed the presence of GRE3 and XYL2 in growing strains and the absence in non-growing strains. It was found that the CaGRE3 possessed an N-terminal extension that improved the functioning of heterologous GRE3/SOR1 combinations. Metabolite analysis showed increased import of xylose when subject to xylose adaption compared to unadapted strains. C. albicans is able to use glucogenic amino acids as a source of carbon, with or without the presence of xylose enzymes. Overall, this work establishes the enzymatic interactions between the xylose reductase and xylose dehydrogenase in S. cerevisiae and C. albicans, as well as their effects on the cell.