In view of the intrinsic redox restrictions associated with the combined in­troduction of xylose reductase and xylitol dehydrogenase into S. cerevisiae, it is relevant to explore alternative metabolic engineering strategies. As will be discussed below, expression of heterologous genes for xylose isomerase (an enzyme that does not naturally occur in S. cerevisiae) offers such an alterna­tive [14]. In the following sections, we will briefly discuss the properties and taxonomic distribution of xylose isomerases. This will be followed by a brief overview of previous attempts at functional expression of xylose isomerases in S. cerevisiae. We will then discuss how, in the past few years, fast progress has been made due to the discovery of a new, fungal xylose isomerase gene. Finally, we will discuss the status of the xylose isomerase strategy with regard to full-scale industrial application.

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