Catalyzes the reversible isomerization of glucose-6-phosphate to fructose-6-phosphate. alpha-D-glucose 6-phosphate = beta-D-fructose 6-phosphate Inhibited by sorbitol-6-phosphate (S6P) and activated by CsrA. Carbohydrate biosynthesis; gluconeogenesis. Carbohydrate degradation; glycolysis; D-glyceraldehyde 3-phosphate and glycerone phosphate from D-glucose: step 2/4. Homodimer. Induced by oxidative stress. One of the main features of pgi mutant is the overproduction of NADPH produced in pentose phosphate (PP) pathway which causes some reducing power imbalance that ultimately affects the cell growth. Cells lacking this gene grow slowly on glucose and utilize glucose primarily via the pentose phosphate pathway as the source of NADPH. They are also hypersensitive to oxidative stress induced by paraquat. Belongs to the GPI family.