CHEMOSTAT CULTIVATION AS A TOOL FOR STUDIES ON SUGAR-TRANSPORT IN YEASTS

Chemostat cultivation enables investigations into the effects of individual environmental parameters on sugar transport in yeasts. Various means are available to manipulate the specific rate of sugar uptake (q(s)) in sugar-limited chemostat cultures. A straightforward way to manipulate q,is variation of the dilution rate, which, in substrate-limited chemostat cultures, is equal to the specific growth rate. Alternatively, q(s) can be varied independently of the growth rate by mixed-substrate cultivation or by variation of the biomass yield on sugar. The latter can be achieved for example, by addition of nonmetabolizable weak acids to the growth medium or by variation of the oxygen supply. Such controlled manipulation of metabolic fluxes cannot be achieved in batch cultures, in which various parameters that ape essential for the kinetics of sugar transport cannot be controlled. In sugar-limited chemostat cultures, yeasts adapt their sugar transport systems to cope with the low residual sugar concentrations, which are often in the micromolar range. Under these conditions, yeasts with high-affinity proton symport carriers have a competitive advantage over yeasts that transport sugars vip facilitated-diffusion carriers. Chemostat cultivation offers unique possibilities to study the energetic consequences of sugar transport in growing cells. For example, anaerobic, sugar-limited chemostat cultivation has been used to quantify the energy requirement for maltose-proton symport in Saccharomyces cerevisiae. Controlled variation of growth conditions in chemostat cultures can be used to study the differential expression of genes involved in sugar transport and as such can make an important contribution to the ongoing studies on the molecular biology of sugar transport in yeasts.