Elevated glucose 6-phosphate levels are associated with plasmid mutations in vivo

The incubation in vitro of plasmid pBR322 DNA with glucose 6-phosphate (Glc-6-P) has been shown to have a mutagenic effect when the plasmid was transformed into wild-type Escherichia coli. To further investigate the modifications of DNA by the reducing sugar Glc-6-P, we have developed an in vivo model to monitor plasmid DNA mutations. E. coli strains that are defective for phosphoglucose isomerase (strain DF40) alone or phosphoglucose isomerase and glucose-6-phosphate dehydrogenase (strain DF2000) accumulate Glc-6-P when grown in gluconate minimal medium in the presence of glucose. These strains and the control strain K10 were transformed with pAM006, a plasmid that carries the genes for ampicillin resistance and beta-galactosidase production, and grown for 24 hr under conditions that prompted the accumulation of Glc-6-P. An increase in plasmid mutations was observed (7- and 13-fold) that was associated with the increased intracellular levels of Glc-6-P (20- and 30-fold) present in the DF40 and DF2000 E. coli strains, respectively. Growth of the mutant bacteria in gluconate minimal medium does not increase the intracellular levels of Glc-6-P or the rate of plasmid mutations over background. Further characterization of the mutated plasmid DNA showed that insertions, deletions, and point mutations were responsible for the loss of beta-galactosidase production. The increase in plasmid mutations as a function of increased intracellular Glc-6-P levels suggests that the accumulation of adducts formed by Glc-6-P and other reducing sugars may contribute to DNA damage.