DUAL MECHANISMS OF XYLOSE UPTAKE IN THE THERMOPHILIC BACTERIUM THERMOANAEROBACTER-THERMOHYDROSULFURICUS

Thermoanaerobacter thermohydrosulfuricus Rt8.B1 catabolized xylose by the pentose phosphate pathway, and xylose isomerase and xylulokinase w ere inducible. The uptake of xylose was by two low-affinity, inducible systems. Both systems were resistant to the protonophore, tetrachloro salicylanilide, the F1F0-ATPase inhibitor, N,N-dicyclohexylcarboiimide , and the sodium/proton antiporter, monensin. The high capacity system (100 nmol min(-1) (mg protein)(-1)) was only expressed when the bacte rium was grown with a high concentration of xylose (50 mM). It took mo re than 60 mM xylose to saturate the high capacity system. When T. the rmohydrosulfuricus was grown with a low concentration of xylose (5 mM) , xylose uptake was saturated by as little as 10 mM xylose (18 nmol mi n(-1) (mg protein)(-1)). Cells grown with 50 mM xylose could not trans port glucose, and high capacity xylose transport was not inhibited by glucose or non-metabolizable glucose analogues. Cells grown with 5 mM xylose transported glucose at a rapid rate (30 nmol min(-1) (mg protei n)(-1)), and low capacity xylose uptake was competitively inhibited by either glucose or 2-deoxy-glucose. Because the glucose uptake of cell s grown on 5 mM xylose was competitively inhibited by xylose, it appea red that the low capacity xylose uptake system was a glucose/xylose ca rrier.