Use of population genetics to derive nonrecombinant Saccharomyces cerevisiae strains that grow using xylose as a sole carbon source.

According to scientific dogma, Saccharomyces cerevisiae cannot grow utilizing xylose as a sole carbon source. Although recombinant DNA technology has overcome this deficiency to some degree, efficient utilization of xylose appears to require complex global changes in gene expression. This complexity provides a significant challenge to the development of yeasts suitable for the utilization of xylose-rich lignocellulosic substrates. In contrast to the dogma, we have found that native strains of S. cerevisiae can grow on xylose as a sole carbon source, albeit very slowly. This observation provided the basis for a new approach using natural selection to develop strains of S. cerevisiae with improved ability to utilize xylose. By applying natural selection and breeding over an extended period, we have developed S. cerevisiae strains that can double in less than 6 h using xylose as a sole carbon source. Strains with improved growth rate possessed increased xylose reductase and xylitol dehydrogenase activities, with the latter showing the greater improvement. This unique, completely nonrecombinant approach to developing xylose-utilizing strains of S. cerevisiae opens an alternative route to the development of yeast that can fully utilize lignocellulosic substrates.