Transport and utilization of hexoses and pentoses in the halotolerant yeast Debaryomyces hansenii

Debaryomyces hansenii is a yeast species that is known for its halotoleranc e, This organism has seldom been mentioned as a pentose consumer. In the pr esent work, a Strain of this species was investigated with respect to the u tilization of pentoses and hexoses in mixtures and as single carbon sources , Growth parameters were calculated for batch aerobic cultures containing p entoses, hexoses, and mixtures of both types of sugars, Growth on pentoses was slower than growth on hexoses, but the values obtained for biomass yiel ds were very similar with the two types of sugars. Furthermore, when mixtur es of two sugars were used, a preference for one carbon source did not inhi bit consumption of the other, Glucose and xylose were transported by cells grown on glucose via a specific low-affinity facilitated diffusion system. Cells derepressed by growth on xylose had two distinct high-affinity transp ort systems for glucose and xylose, The sensitivity of labeled glucose and xylose transport to dissipation of the transmembrane proton gradient by the protonophore carbonyl cyanide m-chlorophenylhydrazone allowed us to consid er these transport systems as proton symports, although the cells displayed sugar-associated proton uptake exclusively in the presence of NaCl or KCl. When the V-max values of transport systems for glucose and xylose were com pared with glucose- and xylose-specific consumption rates during growth on either sugar, it appeared that transport did not limit the growth rate.