Monday, May 2, 2011
Grand Ballroom C-D, 2nd fl (Sheraton Seattle)
Most hemicellulosic hydrolysates contain glucose along with a mixture of other sugars – particularly xylose. This is problematic because virtually all known microbes preferentially ferment glucose, and the presence of glucose represses utilization of other carbon sources. The high levels of xylose found in agricultural residues dictates that this sugar must be efficiently utilized in any economical bioconversion process. The native xylose-fermenting yeast, Spathaspora passalidarum, which was isolated from the gut of passalid beetles, uses glucose and xylose simultaneously under aerobic conditions. Under oxygen limitation, it ferments xylose more rapidly than glucose when each is present as a sole carbon source. To better understand this rare preference for xylose we have probed the physiology of this organism by measuring metabolites and transcripts under various conditions.To analyze the hemicellulosic sugar utilization pathways in wild type S. passalidarum NN245 we conducted fermentations in 2 L bioreactors under four conditions with two variables: aerobic or oxygen limited and with glucose or xylose as carbon sources. Throughout the fermentations, we monitored cell growth, sugar utilization, and product formation. Additionally, we assayed intracellular metabolites and mRNA transcript expression. Key metabolites accumulated to higher concentrations when cells were grown on xylose, which could indicate metabolic bottlenecks, or increased pathway flux. Further, differential analysis of transcript expression by S. passalidarum under these conditions highlighted several candidate genes as new targets for metabolic engineering.