Chiang Mai Journal of Science

Thermotolerant methylotrophic yeast, Ogataea siamensis N22, possesses high yield of xylitol from D-xylose at high temperature.  Xylitol production was compared at 37°C and 40°C in a 2.5L fermenter with 1.5% (v/v) methanol and 100 g/L D-xylose supplementation.  The maximum yield of xylitol (Y_p/s) and volumetric productivity (Q_p) were 0.83 g/g and 0.36 g/L/h at 37°C, respectively; while at 40°C the values appeared to be 0.87 g/g and 0.34 g/L/h, respectively.  The yeast strain was subjected to characterization for XYL1 gene coding for xylose reductase which catalyzes an initial step of xylitol production from D-xylose.  XYL1 gene was cloned from O. siamensis N22 genomic DNA and subsequently sequenced.  As a result, an open reading frame of 960bp encoding 319 amino acids was obtained with a predicted protein molecular mass of 36.3 kD.  Alignment of deduced amino acids sequence indicated that xylose reductase of O. siamensis N22 was closely related to aldose reductase of Candida boidinii with 72% similarity.  O. siamensis N22 recombinant carrying an extra copy of XYL1 integrated into the chromosome showed slightly higher xylose reductase activity and hence xylitiol production than that of the original host strain.