Genetic transformation of Rhodopseudomonas sphaeroides by plasmid DNA

A broad-host-range cloning vector, pUI81, was constructed in vitro from plasmids RSF1010 and pSL25 (a pBR322 derivative) and used to assay for transformation in Rhodopseudomonas sphaeroides. Washing cells with 500 mM Tris was an effective means of inducing competence for DNA uptake. Transformation frequencies as high as 10(-5) (transformants per viable cell) have been achieved by incubating Tris-treated cells with plasmid DNA, 100 mM CaCl2, and 20% polyethylene glycol 6000. Maximum frequencies were obtained when recipient cells were spread onto selective media after a 6.5-h outgrowth period in antibiotic-free medium. The structure (open circular versus closed, covalent circular), size, and concentration of plasmid DNA all significantly affected the transformation frequency. Four different plasmids, all small and suitable as cloning vectors, have been introduced by transformation into several different R. sphaeroides strains. Recombinant DNA carried on small, nonconjugative plasmids with broad host ranges can now be directly transferred to R. sphaeroides by this method.