Abstract
The resistance plasmid NR1 derivative pRR330 consists of a neomycin-kanamycin resistance gene (neo-kan) flanked by directly repeated sequences of both insertion element IS1 DNA (768 base pairs) and 840 base pairs of DNA which are a part of the chloramphenicol acetyltransferase (cam) gene. Most Escherichia coli cell populations that were cultured in high neomycin concentrations carried plasmids whose neo-kan gene amplification was mediated either by IS1 DNA or by cam DNA as homologous recombination sites. This suggests that the final amplified cell populations were the descendants of a single cell.