Homologous sequences other than insertion elements can serve as recombination sites in plasmid drug resistance gene amplification.

A plasmid (pRR983) was constructed which has a gene coding for neomycin and kanamycin resistance flanked by direct repeats of regions of homology which contain no known insertion sequences. pRR983 does not have any homologous IS1 sequences. Growth of Proteus mirabilis harboring pRR983 in medium containing high concentration of neomycin resulted in cells which were highly resistant to both neomycin and kanamycin. Plasmid DNA was analyzed by using restriction endonucleases. In most cases the neomycin resistance gene had been tandemly duplicated by using the homologous DNA sequences flanking the resistance gene as recombination sites. This is analogous to tandem duplication of drug resistance genes on NR1 using the two direct repeats of IS1 as recombination sites. The amplified plasmid DNA returned to its original structure by the deletion of amplified neomycin resistance determinants when the host cells were cultured without selection for high resistance to neomycin.