A 140 base-pair DNA segment situated just upstream of the kanamycin resistance gene of transposon Tn2350, a transposon carried by the plasmid R1, was found to act as an origin of replication and allow autonomous replication of a plasmid composed only of the segment and of the tetracycline resistance gene of pBR322. This segment also promotes site-specific recombination: when cloned in pBR322 it promotes multimer formation in a recA− strain. If two copies are cloned on the same plasmid they promote either deletion or inversion of the intervening region, depending on their orientation relative to each other. DNA gyrase seems to be involved in this process since the inversion rate, in a plasmid carrying sequences in opposite orientations, varies in different nalidixic acid-resistant strains (gyrA mutants) independently isolated.