Evidence for non-enzymatic glycosylation of Escherichia coli chromosomal DNA.

We have recently shown that the process of non-enzymatic glycosylation (glycation) takes place in Escherichia coli under physiological conditions and affects both recombinant and endogenous bacterial proteins. In this study, we further demonstrate that E. coli chromosomal DNA is also subjected to glycation under physiological growth conditions. The E. coli DNA accumulates early glycation (Amadori) products as proven by the nitroblue tetrazolium (NBT) reduction assay. It showed also immunoreactivity to a monoclonal antibody raised against N(in)-(carboxymethyl)lysine and fluorescent properties indicative of modifications with advanced glycation end-products. Two types of fluorophores were detected in the E. coli DNA with excitation maxima at 360 nm and 380 nm and emission maxima at 440 nm and 410 nm. Using the NBT reduction assay, fluorescence spectroscopy and enzyme-linked immunosorbent assay we revealed that glycation adducts accumulate in DNA predominantly in the stationary phase of growth, although they could be detected also in exponential-phase cells. Besides on the growth phase, the extent of DNA glycation depends also on the nutrient broth composition being more extensive in rich media. Thiamine was found to inhibit both DNA glycation and spontaneous point mutations as judged by the decreased rate of the argE3 to Arg(+) reversions in the E. coli strain AB1157.