FIG 6.

Spermidine is involved in DNA cross-linking activity. DNA cross-linking was observed by cultivating the E. coli strains with linearized plasmid DNA. Two hundred nanograms of linearized pUC19 plasmid was added to 1 × 10⁶, 2 × 10⁶, or 4 × 10⁶ bacteria/well of wild-type (WT) E. coli strain DH10B pks⁺, the ΔclbA and ΔspeE mutants, and the complemented derivative. When indicated, 20 μg/ml of spermidine was added to the interaction medium during infection. DNA was then purified, loaded on agarose gel, and run under alkaline denaturing conditions. DNA with covalent interstrand cross-links is nondenaturable and displays delayed migration compared to denatured single-stranded DNA (lower band). This image is representative of three independent experiments.