The composition of plant biomass can vary substantially but all plant biomass is composed of four major polymeric compounds: cellulose (~ 33-51%), hemicellulose (~ 19-34%), pectin (~ 2-20%), and lignin (~ 20-30%) [16, 31]. Upon hydrolysis, plant biomass yields a variety of fermentable hexoses (glucose, 36-50%; mannose, 0.3-12%; galactose, 0.1-2.4%) and pentoses (xy­lose, 3.4-23%; arabinose, 1.1-4.5%). S. cerevisiae can ferment all the hexoses to ethanol, but not the pentoses, which can be a significant portion (25%) of, for example, sugarcane bagasse, a preferred feedstock for bioethanol pro­duction [32]. More than three decades of research have been devoted to the development of yeast for efficient xylose fermentation, initiating with the search for alternative yeasts, such as Pachysolen tannophilus, Pichia stipitis, and Candida shehatae, and in the last two decades focusing on the genetic en­gineering of S. cerevisiae to utilize xylose and arabinose [33]. Please refer to the chapters by Hahn-Hagerdal et al. and van Maris et al. (in this volume) for detailed reviews of this topic [34,35].

After several unsuccessful attempts to produce a functional bacterial xy­lose isomerase in S. cerevisiae, many groups focused for the last decade on efficient expression of fungal xylose utilizing genes and manipulating the pentose phosphate pathway to enhance xylose utilization and fermentation in S. cerevisiae [36]. These research efforts ensured steady but slow progress toward the development of xylose utilizing S. cerevisiae strains, and it was recent successes with the production of a functional Piromyces sp. xylose iso — merase in recombinant S. cerevisiae that opened the way for efficient xylose fermentation by S. cerevisiae at low oxygen levels [37,38]. The main advan­tage of this approach is the circumvention of the redox imbalance problem created by expressing xylose reductase (XYL1) and xylitol dehydrogenase (XYL2) fungal genes in S. cerevisiae. Recognizing the need for S. cerevisiae to ferment all hexoses and pentoses produced during enzymatic hydrolysis of wood, both laboratory and industrial S. cerevisiae platform strains have been developed that can utilize xylose [39] and later co-utilize xylose and arabinose [40].

Apart from monosaccharides, S. cerevisiae can utilize the disaccharides su­crose and maltose, and some Saccharomyces strains can also utilize melibiose and the trisaccharides maltotriose and raffinose [41,42]. However, the major end products of cellulose hydrolysis are cellobiose and cellooligosaccharides, which cannot be utilized by S. cerevisiae. The heterologous expression of four different в-glucosidases in S. cerevisiae was evaluated and the P-glucosidase (BGL1) of Saccharomycopsis fibuligera was found to be produced at the high­est activity levels [43]. Expression of the в-glucosidases encoding genes of Candida wickerhamii, Aspergillus kawachii, and T. reesei yielded activities at least one order of magnitude lower than that of Saccharomycopsis fibuligera. It was shown that multicopy expression of the S. fibuligera BGL1 gene could enable growth on cellobiose as sole carbon source at a rate equivalent to that found on glucose [43,44]. Recently, a S. cerevisiae strain was developed that could utilize both xylose and cellobiose [45].

Even with the introduction of pentose and cellobiose utilizing genes, S. cerevisiae strains preferentially utilize glucose before the other mono — and disaccharides. Deregulation of the strong glucose repression effect in S. cerevisiae would be required to allow cometabolism of sugars derived from plant biomass for high ethanol productivity. Disrupting both the MIG1 and MIG2 genes allowed cometabolism of glucose and sucrose [46], and simi­lar strategies could be used to allow co-utilization of sugars released from plant biomass. Furthermore, simultaneous co-transport of glucose and xylose must be facilitated as the delayed utilization of xylose (in a recombinant xy­lose utilizing strain) is in part an effect of competition for the same glucose transporters in the absence of a xylose-specific transporter [37].

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