On the origin of the chloramphenicol resistance transposon Tn9.

The widely studied chloramphenicol resistance (Cmr) transposon Tn9 came from phage P1Cm0. This phage, however, had acquired its Cmr marker from the R plasmid pSM14. The analysis of the physical structure of pSM14 has now revealed that this plasmid already carried Tn9 and also the tetracycline resistance transposon Tn10. Physical and functional studies indicated that Tn9 of pSM14, although capable of transposition, probably translocated to the P1 genome by reciprocal recombination processes.