Plasmidless, photosynthetically incompetent mutants of Rhodospirillum rubrum

Ethyl methanesulfonate has been shown in the past to render a high percentage of Rhodospirillum rubrum cells plasmidless and photosynthetically incompetent. By probing restriction endonuclease-digested chromosomal DNA from these plasmidless strains with 32P-labeled R. rubrum plasmid DNA, it was shown that no homology exists between the plasmid and the chromosomal DNA of the mutant. Loss of the plasmid in all the nonphotosynthetic isolates was accomplished by the synthesis of spirilloxanthin under aerobic growth conditions, resistance to cycloserine and HgCl2, and loss of ability to grow fermentatively on fructose. Changes in both the protein and lipid composition of the membranes and the impaired uptake of 203HgCl2 in the plasmidless strains (compared with the wild type) suggest either that membrane modification occurs as a result of plasmid loss, accounting for several of the acquired phenotype characteristics of the cured strains, or that both membrane modification and plasmid loss are part of the same pleiotropic mutation. 24 references, 7 figures, 3 tables