Heteroduplex mapping of small plasmids derived from R-factor R12: in vivo recombination occurs at IS1 insertion sequences.

Small, autonomously replicating plasmids derived by in vivo recombination from R-factor R12 (= R100) have been structurally mapped by heteroduplex formation between the plasmids and an R-factor which is structurally closely related to R6-5. Recombination resulting in generation of the small resistance-free plasmids occurs between the (IS1)b insertion sequence and various other sites on the opposite side of an origin of replication. A larger R12-derived plasmid pSM17, carrying streptomycin (Sm), sulfadiazole (Sa), and chloramphenicol (Cm) resistances, has recombined in a similar manner but at the (IS1)a sequence. A new structural coordinate origin for R100 and for partially homologous R-factors is proposed based upon the location of the (IS1)b sequence.