Abstract
Many of the known xylose-fermenting (X-F) yeasts are placed in the Scheffersomyces clade, a group of ascomycete yeasts that have been isolated from plant tissues and in association with lignicolous insects. We formally recognize fourteen species in this clade based on a maximum likelihood (ML) phylogenetic analysis using a multilocus dataset. This clade is divided into three subclades, each of which exhibits the biochemical ability to ferment cellobiose or xylose. New combinations are made for seven species of Candida in the clade, and three X-F taxa associated with rotted hardwood are described: Scheffersomyces illinoinensis (type strain NRRL Y-48827T = CBS 12624), Scheffersomyces quercinus (type strain NRRL Y-48825T = CBS 12625), and Scheffersomyces virginianus (type strain NRRL Y-48822T = CBS 12626). The new X-F species are distinctive based on their position in the multilocus phylogenetic analysis and biochemical and morphological characters. The molecular characterization of xylose reductase (XR) indicates that the regions surrounding the conserved domain contain mutations that may enhance the performance of the enzyme in X-F yeasts. The phylogenetic reconstruction using XYL1 or RPB1 was identical to the multilocus analysis, and these loci have potential for rapid identification of cryptic species in this clade.
Highlights
IntroductionOverexpression, homologous and heterologous expression, and direct mutagenesis of genes involved in D-xylose assimilation and fermentation have only modestly enhanced the quantity of ethanol production by yeasts due to several metabolic constraints; these include rate of regeneration of the cofactor NADP(H) required by xylose reductase (XR) and xylitol dehydrogenase (XDH), repression by glucose, and anaerobic respiration regulatory control [15,16,17]
D-xylose is a five-carbon backbone molecule of the hemicellulose component of plant cell walls and is one of the most abundant renewable carbon resources on Earth
In order to ferment D-xylose, yeasts express xylose reductase (XR), xylitol dehydrogenase (XDH), and xylulose kinase (XK) to convert D-xylose to Dxylulose-5-phosphate; D-xylulose-5-phosphate is incorporated into the pentose phosphate pathway to be catalyzed to ethanol [2,3,4]
Summary
Overexpression, homologous and heterologous expression, and direct mutagenesis of genes involved in D-xylose assimilation and fermentation have only modestly enhanced the quantity of ethanol production by yeasts due to several metabolic constraints; these include rate of regeneration of the cofactor NADP(H) required by XR and XDH, repression by glucose, and anaerobic respiration regulatory control [15,16,17]. These studies have resulted in the present understanding of the biochemical pathway, but the main goal of bioengineering yeasts capable of fermenting D-xylose at a high rate to be used at industrial scales has not yet been achieved. Recent research has been focused on the discovery of new X-F yeasts, e.g. Spathaspora passalidarum and Candida jeffriesii [18], and Spathaspora arborariae and other taxa [19,20], from the guts of lignicolous beetles and rotted wood, niches from which a number of X-F yeasts have been isolated [21,22,23]
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