Stability of DNA repeats in Escherichia coli dam mutant strains indicates a Dam methylation-dependent DNA deletion process.

In this study we report on the stabilization of short direct repetitive DNA elements. We arranged a 20 bp SK-primer element in a direct repeat manner within the cloning vector pBluescript KS (+). This resulted in an array of 27 direct repeats consisting of 24 bp units. We show that this plasmid could only be propagated without deletion of repeats in dam mutant Escherichia coli hosts, whereas all efforts to use strains that were defective in the methylation-dependent restriction system and the recA- or the mismatch repair-dependent deletion system failed. The deletions always affected whole repeat units and not parts of them, leading to an unpredictable reduction of the unit number down to a range of between 12 and two during propagation. We conclude that a Dam methylation-dependent, but recA- and mismatch repair-independent, deletion mechanism caused the DNA rearrangements without an obvious involvement of the known methylated-DNA restriction systems.