A hypothetical model of C-terminal tip displacement. Homologous recombination between the conserved core sequence within the downstream unattached tip and either the core region of the rhs gene or that of another unattached tip. This event would result in the production of a recombination proficient episomal circle carrying a conserved core region, of varying lengths, which is attached to a variable C-terminal tip and any intervening genes. After transfer to a second bacterial isolate homologous recombination would be required between the highly conserved core regions on the chromosomal rhs gene and identical sequences located on the episomal circle carrying the unattached tip. This second single cross-over event of a circular intermediate is required to explain how an attached tip can be displaced by a new tip without deleting or entirely replacing the old tip, but simply shunting it to a silent position downstream of the intact rhs gene.