Characterization of a Rhodospirillum rubrum plasmid: loss of photosynthetic growth in plasmidless strains.

A single plasmid of 55 kilobases was found in crude cell lysates of each of nine strains of Rhodospirillum rubrum. Restriction endonuclease analysis showed identical fragment patterns with a given nuclease for all plasmids except one, for which an additional EcoRI site was observed. Elimination of the plasmid required that the cells be passaged several times in 25 mM calcium-containing medium, followed by at least two passages under photosynthetic growth conditions in low-calcium medium before treatment with ethyl methanesulfonate. The resulting plasmidless mutants only grew aerobically and were all incapable of pigment formation and photosynthetic growth, suggesting that plasmid DNA is required for photosynthetic competence in R. rubrum.