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Plaque-Forming Recombinants between Bacteriophage P1 and R factor NR1 and their Behaviour in Lysogenic Conditions K. Katsutoshi Mise W. Werner Arber Author’s address: Prof. Dr. W. Arber, Abteilung Mikrobiologie, Biozentrum der Universität Basel, Klingelbergstrasse 70, CH-4056 Basel (Switzerland) The progeny of bacteriophage PI propagated in a strain of Escherichia coli harboring the R factor NRl was submitted to a stepwise enrichment for plaque-forming PI derivatives which transduced drug resistance determinants at high frequency. From these preparations PICM and PITC phage strains were isolated which carry determinants for resistance to chloramphenicol and to tetracycline, respectively, in their genomes. By recombination between PICM and PITC doubly resistant PICMTC was also obtained. Various properties of a few of these plaque-forming transducing phages were compared with those of wild type PI. All derivatives tested reproduce vegetatively with the same latent period as PI. Some of them give a smaller burst size and form smaller plaques than PI. The phage particles of some of the derivatives giving small plaques have a higher buoyant density than PI. All PI derivatives studied lysogenize well and the prophage is stably propagated by the lysogenic strains produced. With PICM and PITC the formation and the properties of rare doubly lysogenic strains were studied after selection of CM1’ TCr colonies on medium containing the two antibiotics. Some of the stable strains thus isolated carried a PICMTC prophage while others could not yet be characterized unequivocally. Apparently the most frequent CMr TO” strains are unstable double lysogens and segregate in the absence of the antibiotics either single lysogens carrying one or the other of the two prophages or nonlysogenic bacteria having lost all detectable prophage material.