KINETICS OF SUGAR-TRANSPORT AND PHOSPHORYLATION INFLUENCE GLUCOSE ANDFRUCTOSE COMETABOLISM BY ZYMOMONAS-MOBILIS

The competitive inhibition of fructokinase by glucose has been propose d as the mechanism by which Zymomonas mobilis preferentially consumes glucose from mixtures of glucose and fructose and accumulates fructose when growing on sucrose. In this study, incorporation of radioactive fructose into biomass was used as a measure of fructose catabolism. It was determined that the rate of fructose incorporation by Z. mobilis CP4 was somewhat lower in the presence of an equimolar concentration o f glucose but that the inhibition of fructokinase by glucose was not n early as severe in vivo as was predicted from in vitro studies. Intere stingly, addition of glucose to a culture of Z. mobilis CP4-M2, a gluc okinaseless mutant, resulted in an immediate and nearly complete inhib ition of fructose incorporation. Furthermore, addition of nonmetaboliz eable glucose analogs had a similar effect on fructose catabolism by t he wild-type Z. mobilis CP4, and fructose uptake by Z. mobilis CP4-M2 was shown to be severely inhibited by equimolar amounts of glucose. Th ese results suggest that competition for fructose transport plays an i mportant role in preferential catabolism of glucose from sugar mixture s. Indeed, the apparent K-m values for sugar uptake by Z. mobilis CP4 were approximately 200 mM for fructose and 13 mM for glucose. Other ex periments supported the conclusion that a single facilitated diffusion transport system, encoded by the glf gene, is solely responsible for the uptake of both glucose and fructose. The results are discussed wit h regard to the hypothesis that the kinetics of sugar transport and ph osphorylation allow the preferential consumption of glucose and accumu lation of fructose, making the fructose available for the enzyme gluco se-fructose oxidoreductase, which forms sorbitol, an important osmopro tectant for Z. mobilis when growing in the presence of high sugar conc entrations.