Pichia Stipitis

''Scheffersomyces stipitis'' (formerly ''Pichia stipitis'') is a species of yeast, belonging to the "CUG Clade" of ascomycetous yeasts. This is a group of fungi that substitute serine for leucine when the CUG codon is encountered. ''S. stipitis'' is distantly related to brewer's yeast, '' Saccharomyces cerevisiae'', which uses the conventional codon system. Found, among other places, in the guts of passalid beetles, ''S. stipitis'' is capable of both aerobic and oxygen limited fermentation, and has the highest known natural ability of any yeast to directly ferment xylose, converting it to ethanol, a potentially economically valuable trait. Xylose is a hemicellulosic sugar found in all angiosperm plants. As such xylose constitutes the second most abundant carbohydrate moiety in nature. Xylose can be produced from wood or agricultural residues through auto- or acid hydrolysis. Ethanol production from such lignocellulosic residues does not compete with food production through the consumption of grain.

Given the abundance of xylose and its potential for the bioconversion of lignocellulosic materials to renewable fuels, ''Scheffersomyces stipitis'' has been extensively studied. The complete sequencing of its genome was announced i
2007

Native strains of ''S. stipitis'' have been shown to produce ≈50 g/L ethanol in 48 h from pure xylose in defined minimal medium using urea as a nitrogen source. ''S. stipitis'' is a predominantly haploid yeast but strains can be induced to mate with themselves or with other strains of ''S. stipitis'' by cultivating cells on minimal medium containing limiting amounts of carbon sources and nitrogen. An extensive genetic toolbox has been developed for ''S. stipitis'' that includes synthetic drug resistance markers fo

and a synthetic form of Cre that enables excision of the markers. Engineered strains of ''S. stipitis'' will produce 57 g/L ethanol from pure xylose in under 48 h and adapted strains will produce significant amounts of ethanol from acid hydrolysates of lignocellulose.

This natural ability of ''S. stipitis'' to ferment xylose to ethanol, has inspired efforts to engineer this trait into ''Saccharomyces cerevisiae''.
''S. cerevisiae'' is preferred for ethanol production from grain and sugar cane, because it ferments hexose sugars very rapidly and is very robust. However, it does not natively metabolize xylose. This limits the usefulness of ''S. cerevisiae'' in the production of fuels and chemicals from plant cell walls, which contain a large amount of xylose. In response, ''S. cerevisiae'' has been engineered to ferment xylose through the addition of the ''S. stiptis'' genes, XYL1 and XYL2, coding for xylose reductase and xylitol dehydrogenase, respectively. The concerted action of these enzymes converts xylose to xylulose, which is naturally fermented by ''S. cerevisiae''. Additional modifications are necessary for rapid fermentation of xylose, however.

References

{{Taxonbar, from=Q10663215

Saccharomycetaceae
Yeasts