Another factor that was shown to effect the activity of XDH is the promoter that is used in the cloning, and the coexpression of the XYL1 gene, encoding for xylose reductase, as thoroughly investigated by Walfridsson et al (33). Higher activities were obtained for XYL2 that was controlled by the PGK1 promoter, when XYL1 and XYL2 were transcribed in the same direction with XYL2 downstream of XYL1 and the ratio of XR:XDH was 0.06 (33). They also reported that the presence of the ADH1 promoter-terminator fragment enhanced the activity of the structural gene expressed under the control of the PGK1 promoter up to 20-fold (33). The highest specific activities of XR and XDH were
observed when the ADH1 promoter-gene-terminator was placed upstream of the PGK1 promoter-gene-terminator (33). In this work, XYL2 was placed downstream of the selective marker gene SFA, by which both genes were controlled by one promoter, the ADH2 promoter. This construct could cause the low activity of XDH, in addition to the use of glucose containing medium. Since this is a preliminary result, further optimizations such as changing carbon sources or the cloning strategy by using appropriate promoter are needed to be done in order to enhance the activity of xylitol dehydrogenase
from C. shehatae in S. cerevisiae. Nevertheless, this work demonstrates the functionality of the gene xylitol dehydrogenase from C. shehatae in S. cerevisiae since the cloning of the XYL2 gene of C. shehatae in S. cerevisiae has not been reported previously.