To investigate the physiological basis of decreased rate of glucose utilization by Thiobacillus novellus in a mixotrophic environment (R. C. Perez and A. Matin, J. Bacteriol. 142:633–638, 1980), its glucose transport system was characterized and the modulation of this system as well as enzymes of glucose metabolism by the growth environment was examined. Uptake of 2-deoxy-d-glucose by cell suspensions was almost abolished by respiratory chain inhibitors, and the sugar accumulated unchanged inside the cells against a concentration gradient: its transport is probably linked to the proton electrochemical gradient. The glucose transport system, as well as several enzymes of glucose metabolism, had a high specific activity in heterotrophic cells, intermediate activity in mixotrophic cells, and low activity in autotrophic cells; thus, they are induced by glucose but repressed by thiosulfate, its metabolites, or both. Thiosulfate and sulfite inhibited the glucose transport system uncompetitively and noncompetitively, respectively (apparent K_i = 3.1 × 10⁻² M and 3.3 × 10⁻⁷ M, respectively) and also inhibited glucose-6-phosphate dehydrogenase activity. Thus, the rate of glucose utilization in mixotrophic environments decreased because thiosulfate and its metabolites repress as well as inhibit the glucose transport system and enzymes of glucose metabolism. The significance of this and other regulatory phenomena that come into play in such environments is discussed.

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