Summary: The properties of four P-group plasmids, R26, R527, R751 and R906, which differ in resistance phenotype or in the bacterial species in which they were first detected, have been compared with the prototype of this group, RP1. Two of the plasmids, R26 and R527, are new isolates which have been assigned to the P group because of their incompatibility with R751. The properties studied include response to aeruginocin and to male and female sex-specific phages, interaction with prophage B3 and fertility inhibition by plasmid R38. Strains harbouring these plasmids behaved similarly in all tests except those involving aeruginocin. This suggests that the locus for aeruginocin-insensitivity is one of the R-determinants whereas the genes controlling the remaining characteristics are closely linked to the transfer factor. These plasmids may therefore have a common ancestor and their differences in resistance phenotype may simply reflect recombination events which they have undergone subsequently. Their similarity is also seen in transduction experiments, since determinants from two of these plasmids can be ‘rescued’ by the P-group plasmid R18 if this is already present in the recipient cell and the host recombination system is functional.


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