Summary: Three phages specific for bacteria harbouring the P-group plasmid RP1 have been isolated, and their properties compared with those of a previously described sex-specific phage, PRR1 (Olsen & Shipley, 1973). These phages are distinguishable from each other by various criteria, although in terms of host range to FP and RP lines of they comprise broadly two groups. Thus all the phages infect bacteria harbouring any of a group of plasmids with similar properties to those of RP1, but whereas the filamentous phage Pf3 is specific for this group, the host ranges of PRR1, PR3 and PR4 are considerably wider. Nevertheless, with one exception, this does not extend to plasmids isolated outside the United Kingdom, which suggests that all these plasmids share a common ancestry even though by other criteria they constitute three fairly discrete subgroups. Of the plasmids that fail to allow phage propagation, three, when present in the same cell as RP1, reduce its susceptibility to phage infection. This inhibition may reflect a relationship between these elements, similar to that found among plasmids of Enterobacteria. A correlation is observed between the susceptibility of bacteria to phage infection and their ability to mediate plasmid transfer, such that these phages can conveniently be used to isolate both derepressed or transfer-defective mutants of various R factors.


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