P22 mediated recombination of frt-sites

Flp mediated site specific recombination of frt-sites is frequently used in genetic engineering to excise, insert or invert DNA-cassettes in the chromosome. While constructs flanked by frt-sites are generally considered to be stable in the absence of the Flp enzyme, we observed that P22 chromosomes exceeding wild-type length tend to lose frt-flanked insertions via Flp independent recombination of frt-sites during phage propagation. This spontaneous recombination should be considered when engineering the chromosome of P22 and perhaps of other phages as well.