Localization of Phosphoglucose Isomerase in Escherichia coli and Its Relation to the Induction of the Hexose Phosphate Transport System

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RESUMO

The localization of phosphoglucose isomerase (PGI) was studied in relation to the induction of hexose phosphate uptake in Escherichia coli. The uptake system is induced only by extracellular glucose-6-phosphate (G6P); there is no induction by intracellular G6P. Fructose-6-phosphate (F6P) is an indirect inducer, and isomerization of F6P to G6P must occur before induction. PGI has been considered to be an internal enzyme; therefore, uptake of F6P by noninduced cells and leakage of the G6P formed would be required for induction. In this study, it was concluded that part of the PGI activity is located in the cell surface because: (i) uninduced, intact cells are able to convert F6P to G6P, whereas the activity of G6P dehydrogenase is not detectable; (ii) when cells are subjected to osmotic shock, about 10% of the PGI activity is found in the shock fluid; and (iii) sorbitol-6-phosphate (S6P) inhibits both PGI activity of whole cells and the induction of hexose phosphate transport system by F6P. S6P was not taken by intact cells. The data indicate that the isomerization of F6P to G6P can take place on the cell surface, and this explains the indirect induction of hexose phosphate transport by F6P.

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