Reversible inactivation of d-xylose utilization by d-glucose in the pentose-fermenting yeast Pachysolen tannophilus

A major problem in fermenting pentoses using lignocellulosic substrates is the presence of d-glucose which inhibits d-xylose utilization. We previously showed that d-glucose represses the induction of xylose reductase and xylitol dehydrogenase activities, thereby inhibiting d-xylose utilization in Pachysolen tannophilus. The question arose whether d-glucose can also inactivate d-xylose fermentation. P. tannophilus cells were grown on a defined d-xylose-containing liquid medium. At about 40 h, d-glucose was added to a final concentration of 3% (w/v). This led to a rapid cessation of d-xylose utilization, which resumed after 10–12 h before d-glucose was completely consumed. This suggests that d-glucose inactivated existing d-xylose catabolic enzymes and that inactivation was reversed at low d-glucose concentrations. This reversible inactivation was distinct from d-glucose repression. Addition of cycloheximide did not block the resumption of d-xylose consumption, suggesting that reactivation was independent of protein synthesis.