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

Chemostat cultivation enables investigations into the effects of indiv idual environmental parameters on sugar transport in yeasts. Various m eans are available to manipulate the specific rate of sugar uptake (q( s)) in sugar-limited chemostat cultures. A straightforward way to mani pulate q,is variation of the dilution rate, which, in substrate-limite d chemostat cultures, is equal to the specific growth rate. Alternativ ely, q(s) can be varied independently of the growth rate by mixed-subs trate cultivation or by variation of the biomass yield on sugar. The l atter can be achieved for example, by addition of nonmetabolizable wea k acids to the growth medium or by variation of the oxygen supply. Suc h controlled manipulation of metabolic fluxes cannot be achieved in ba tch cultures, in which various parameters that ape essential for the k inetics of sugar transport cannot be controlled. In sugar-limited chem ostat cultures, yeasts adapt their sugar transport systems to cope wit h the low residual sugar concentrations, which are often in the microm olar range. Under these conditions, yeasts with high-affinity proton s ymport carriers have a competitive advantage over yeasts that transpor t sugars vip facilitated-diffusion carriers. Chemostat cultivation off ers unique possibilities to study the energetic consequences of sugar transport in growing cells. For example, anaerobic, sugar-limited chem ostat cultivation has been used to quantify the energy requirement for maltose-proton symport in Saccharomyces cerevisiae. Controlled variat ion of growth conditions in chemostat cultures can be used to study th e 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.