PLEIOTROPIC REGULATION OF CENTRAL CARBOHYDRATE-METABOLISM IN ESCHERICHIA-COLI VIA THE GENE CSRA

The carbon storage regulator gene csrA has been shown previously to dr amatically affect the biosynthesis of intracellular glycogen in Escher ichia coli through its negative control of the expression of two glyco gen biosynthetic operons and the gluconeogenic gene pckA (Romeo, T., G ong, M., Liu, M. Y., and Brun-Zinkernagel, A. M, (1993) J, Bacteriol, 175, 4744-4755). Examination of the effects of csrA on several enzymes , genes, and metabolites of central carbohydrate metabolism now estab lishes a more extensive role for csrA in directing intracellular carbo n flux. Phosphoglucomutase and the gluconeogenic enzymes fructose 1,6- bisphosphatase and phosphoenolpyruvate synthetase were found to be und er the negative control of csrA, and these enzyme activities were maxi mal during the early stationary phase of growth. The enzymes glucose-6 -phosphate isomerase, triose-phosphate isomerase, and enolase were pos itively regulated by csrA. Thus, csrA exerts reciprocal effects on gly colysis versus gluconeogenesis and glycogen biosynthesis. The glycolyt ic isozymes pyruvate kinase F and A (encoded by pykF and pykA, respect ively) and phosphofructokinase I and II (pfkA and pfkB, respectively) exhibited differential regulation via csrA. Since the individual membe rs of these isozyme pairs are allosterically regulated by different ce llular metabolites, csrA is also capable of fine-tuning the allosteric regulation of glycolysis. In contrast, the expression of genes of the pentose phosphate pathway was weakly or negligibly affected by csrA.