STRAIN SELECTION, TAXONOMY, AND GENETICS OF CLOSE-FERMENTING YEASTS

Xylose utilization is essential for the efficient conversion of lignoc ellulose to ethanol. The objective of this review is to trace the deve lopment of xylose-fermenting yeast strains from their discovery in 198 0. Following initial reports, screens of known yeasts identified five species of interest: Candida shehatae, Candida tenuis, Pachysolen tann ophilus, Pichia segobiensis, and Pichia stipitis. Candida shehatae str ains can be divided into three varieties. Pachysolen tannophilus and P ichia stipitis have been studied most extensively and have the best-un derstood genetic systems. Improved mutants of P. tannophilis have been obtained by selecting for an inability to oxidize ethanol (eth) and f or rapid growth on xylitol and nitrate. Improved P. stipitis mutants h ave been obtained by selecting for flocculation, decreased utilization of glucose, and growth on noninductive carbon sources. Bacterial xylo se isomerase has been cloned and expressed in S. cerevisiae and Schizo saccharomyces pombe, but the heterologous enzyme is inactive. Xylose r eductase and xylitol dehydrogenase have been cloned from P. stipitis a nd expressed in Saccharomyces cerevisiae, giving rise to transformant S. cerevisiae that grow on xylose but that ferment it poorly. A transf ormation and expression system based on the URA3 marker has recently b een developed for P. stipitis so that contemporary genetic methods may be brought to bear on this organism.