Abstract
The ascomycetes Candida albicans, Saccharomyces cerevisiae and Scheffersomyces stipitis metabolize the pentose sugar xylose very differently. S. cerevisiae fails to grow on xylose, while C. albicans can grow, and S. stipitis can both grow and ferment xylose to ethanol. However, all three species contain highly similar genes that encode potential xylose reductases and xylitol dehydrogenases required to convert xylose to xylulose, and xylulose supports the growth of all three fungi. We have created C. albicans strains deleted for the xylose reductase gene GRE3, the xylitol dehydrogenase gene XYL2, as well as the gre3 xyl2 double mutant. As expected, all the mutant strains cannot grow on xylose, while the single gre3 mutant can grow on xylitol. The gre3 and xyl2 mutants are efficiently complemented by the XYL1 and XYL2 from S. stipitis. Intriguingly, the S. cerevisiae GRE3 gene can complement the Cagre3 mutant, while the ScSOR1 gene can complement the Caxyl2 mutant, showing that S. cerevisiae contains the enzymatic capacity for converting xylose to xylulose. In addition, the gre3 xyl2 double mutant of C. albicans is effectively rescued by the xylose isomerase (XI) gene of either Piromyces or Orpinomyces, suggesting that the XI provides an alternative to the missing oxido-reductase functions in the mutant required for the xylose-xylulose conversion. Overall this work suggests that C. albicans strains engineered to lack essential steps for xylose metabolism can provide a platform for the analysis of xylose metabolism enzymes from a variety of species, and confirms that S. cerevisiae has the genetic potential to convert xylose to xylulose, although non-engineered strains cannot proliferate on xylose as the sole carbon source.
Highlights
Sugars represent an important source of carbon and energy for fungi
The baker’s or brewer’s yeast S. cerevisiae, the fungal pathogen C. albicans, and the pentose-sugar fermenting yeast S. stipitis are related ascomycetes, with C. albicans and S. stipitis in a lineage that uses the codon CTG to encode serine, unlike S. cerevisiae, which, consistent with the universal genetic code, uses CTG to encode leucine. These ascomycetes have distinct ecological niches; S. cerevisiae is almost a domesticated organism primarily associated with human brewing and baking, C. albicans is a human commensal colonizing mucosal surfaces and the gastrointestinal system, while S. stipitis inhabits the guts of some insects
We have extended these studies to show that when put in the common context of the C. albicans cell, genes for xylose metabolism from both the xylose metabolizing yeast S. stipitis and the nonutilizing yeast S. cerevisiae are able to complement the deletion of these functions and permit the growth of mutant C. albicans strains on xylose medium
Summary
Sugars represent an important source of carbon and energy for fungi. different species can have widely differing capacities to use sugars for growth. Fungal cells prefer to use the 6-carbon sugar glucose, which they can convert to pyruvate and oxidize to CO2 and water with the concomitant production of up to 36 ATP units through the process of oxidative phosphorylation [1]. Because of this efficiency, many cells suppress the metabolism of other sugars in the presence of glucose [2]. Disaccharides and more complex sugars can be enzymatically converted to monosaccharides and to glucose/ fructose for entry into the glycolytic pathway [5,6,7]
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