Properties of the viral R plasmid R46P are described. As a temperate bacteriophage, it plaques on the facultative phototroph Rhodopseudomonas sphaeroides. Under aerobic conditions the phage had a latent period of 180 min, a burst time of 200 min, and a burst size of 15 to 20 particles per infective center. The encapsidated viral genome occurred as a supercoiled, circular DNA duplex with a mean contour length of 16.5 ± 1.0 ,um. Percent guanine plus cytosine, as calculated from thermal

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... n profiles, was 63.5. Mitomycin C-induced loss of the prophage suggested an extrachromosomal location in the host cell. Use of this curing agent enabled the isolation of a plasmid-free strain of R. sphaeroides. Biophysical analysis of the plasmid-free strain lysogenized with RE6P confirmed that the prophage occurred as a plasmid in the host cell. 1.5% (wt/vol) agar. PYE layer agar consisted of PYE broth, 0.01 M MgSO4, and 0.8% (wt/vol) agar. TMG buffer contained 0.01 M tris(hydroxymethyl)amninomethane, 0.01 M MgSO4, and 0.01% (wt/vol) Difco bacteriological gelatin, adjusted to pH 7.4 before autoclave sterilization. All cultures were grown at 32°C unless otherwise stated. Liquid cultures were aerated by shaking at 350