(409d) Xylose Fermentation and Respiratory-Deficiency in Saccharomyces Cerevisiae

For several practical reasons, the yeast Saccharomyces cerevisiae is the preferred workhorse for industrial bioethanol production, however this organism lacks native pathways to utilize xylose – one of the more abundant sugars in lignocellulosic biomass.  Heterologous expression of xylose-assimilation pathways confers the ability to metabolize this substrate upon S. cerevisiae, though several studies report such recombinant yeasts exhibit a respiratory-response to this carbon-source.  It has been thought that respiratory-deficient strains cannot grow on xylose in spite, quite remarkably, of their ability to ferment it, however a recent study demonstrated that extensive adaptive evolution can confer growth phenotypes on these strains.  As such, balancing characteristics of respiratory and fermentative metabolism may prove crucial for efficient xylose fermentation. 

We hope to elucidate the xylose metabolism of recombinant S. cerevisiae in the context of respiration and fermentation.  Our approach involves the isolation and characterization of mutants from genomic and respiration-rescuing libraries expressed in petite backgrounds (respiratory-deficient yeasts).  The most desired mutants include strains which exhibit high fermentation rates while limiting the loss of carbon to respiration and production of biomass.  Such strains are difficult to isolate from diverse libraries by virtue of their growth characteristics.  To address such challenges, we have developed a method employing microfluidics capable of measuring the production and consumption of bulk-phase metabolites on a single-cell basis.  We have previously demonstrated the application of this approach in order to identify genomic changes that result from adaptive evolution in xylose-consuming S. cerevisiae which confer superior xylose assimilation.  By extending the use of this platform to the study of genomic and respiratory libraries expressed in petite backgrounds, we hope to identify targets for inverse metabolic engineering that will allow for the rational design of strain which exhibit superior xylose-fermentation characteristics.