SUMMARY: PS18(RP1) is resistant to carbenicillin, neomycin/kanamycin and tetracycline. This strain gave rise to a derivative: PS18(RP1-1), resistant only to carbenicillin. RP1 was transmissible from PS18(RP1) to other strains of or while RP1-1 was transmissible to other strains of could not be transferred to and transferred only at a very low frequency to DNA hybridization showed that PS18(RP1-1) contained approximately one sixth as much RP1-homologous DNA as PS18(RP1). Although satellite DNA could be detected in strains carrying RP1 by ethidium bromide/CsCl-gradient analysis and by analytical CsCl-gradient centrifuging, no such satellite DNA could be found in strains of carrying RP1-1 or carrying a non-transferable derivative RP1-1. Drug resistance could be eliminated from RP1-containing strains by treatment with sodium dodecyl sulphate, but the carbenicillin resistance of RP1-1 strains could not be eliminated. The apparent absence of satellite DNA in bacteria carrying RP1-1 or RP1-1, and the failure to ‘cure’ them of their carbenicillin resistance suggests the possibility that the RP1-1 and RP1-1 genes might be a fragment of RP1 integrated in the chromosome of the strains of containing RP1-1 and containing RP1-1.


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