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Summary: Caulobacter strains which contain plasmid RP1 are resistant to carbenicillin, kanamycin and tetracycline and can serve as donors of the plasmid, but such R+ strains are not susceptible to the RP1-specific phage PRR1. Since both conjugation and phage PRR1 infection are mediated by the plasmid-specified pilus, the lack of phage binding suggests some functional alteration of the pilus. In this report we demonstrate that RP1 pili are produced by Caulobacter R+ cells, but unlike the RP1 pili produced by PRR1-susceptible strains, the Caulobacter R+ pili do not inactivate phage PRR1. Antibody produced against Caulobacter R+ pili prevents phage PRR1 binding to RP1 pili from PRR1-susceptible strains of Pseudomonas aeruginosa and Escherichia coli, and also inhibits plasmid transfer by those strains. Antibody produced against RP1 pili from P. aeruginosa and E. coli inhibits plasmid transfer both by those strains and by Caulobacter R+ donors. Phage PRR1 rendered non-infectious by RNAase treatment prevents plasmid transfer by P. aeruginosa and E. coli but not by Caulobacter.