Dynamic response of the expression of hxt1, hxt5 and hxt7 transport proteins in Saccharomyces cerevisiae to perturbations in the extracellular glucose concentration.

Glucose transport in Saccharomyces cerevisiae relies on a multi-factorial uptake system. The modulation of its efficiency depends on the differential expression of various sets of hexose transport-related proteins whose glucose affinity differs considerably. The expression of three different glucose transport proteins (HXT1, HXT5 and HXT6/7 with low-, intermediate- and high-affinity, respectively) was monitored as a result of modified extracellular glucose concentrations. Cultivation at glucose-limited (continuous) conditions was instantly replaced by a batch (and thus, non-limited) mode. Further, to mimic concentration gradients in large-scale production bioreactors, multiple and rapid transient glucose pulses were applied to chemostat cultivation. Antibodies against the HXT-proteins were used to monitor the proteins' expression levels prior to and after perturbing the external glucose concentrations. HXT5 and HXT6/7 were either expressed during the starvation-like steady-state phases in the chemostat cultivations, whereas HXT1 could not be detected at all. HXT1, however, is subsequently expressed during the excess of glucose in the batch mode, while the HXT5 and HXT6/7 transporters were at least found to decline. These findings coincide well with the transporters' affinity profiles. As a result of repeated and rapid transient glucose pulses during continuous fermentation, especially HXT6/7 pointed out to alter the protein expression pattern.