Conjugation and chromosome transfer inRhodopseudomonas sphaeroidesmediated by W and P group plasmids

Comparison of the nucleotide and amino acid sequences of the RsrI and EcoRI restriction endonucleases

The RsrI endonuclease, a type-II restriction endonuclease (ENase) found in Rhodobacter sphaeroides, is an isoschizomer of the EcoRI ENase. A clone containing an 11-kb BamHI fragment was isolated from an R. sphaeroides genomic DNA library by hybridization with synthetic oligodeoxyribonucleotide probes based on the N-terminal amino acid (aa) sequence of RsrI. Extracts of E. coli containing a subclone of the 11-kb fragment display RsrI activity. Nucleotide sequence analysis reveals an 831-bp open reading frame encoding a polypeptide of 277 aa. A 50% identity exists within a 266-aa overlap between the deduced aa sequences of RsrI and EcoRI. Regions of 75-100% aa sequence identity correspond to key structural and functional regions of EcoRI. The type-II ENases have many common properties, and a common origin might have been expected. Nevertheless, this is the first demonstration of aa sequence similarity between ENases produced by different organisms.

Purification and characterization of the restriction endonuclease RsrI, an isoschizomer of EcoRI

Rhodobacter sphaeroides strain 630 produces restriction enzyme RsrI which is an isoschizomer of EcoRI. We have purified this enzyme and initiated a comparison with the EcoRI endonuclease. The properties of RsrI are consistent with a reaction mechanism similar to that of EcoRI: the position of cleavage within the -GAATTC-site is identical, the MgCl2 optimum for the cleavage is identical, and the pH profile is similar. Methylation of the substrate sequence by the EcoRI methylase protects the site from cleavage by the RsrI endonuclease. RsrI cross-reacts strongly with anti-EcoRI serum indicating three-dimensional structural similarities. We have determined the sequence of 34 N terminal amino acids for RsrI and this sequence possesses significant similarity to the EcoRI N terminus.

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.

High-frequency chromosome transfer in Rhodopseudomonas sphaeroides promoted by broad-host-range plasmid RP1 carrying mercury transposon Tn501

Insertion of the mercury resistance transposon Tn501 into broad-host-range plasmid RP1 greatly enhanced the ability of this plasmid to promote chromosome transfer in the photosynthetic bacterium Rhodopseudomonas sphaeroides. Compared with the wild-type RP1, which produced less than 10(-8) recombinants per donor cell, RP1::Tn501 produced between 10(-3) and 10(-7) recombinants per donor cell depending upon the marker selected. Plasmid RP1::Tn501 promoted polarized transfer of the chromosome from one or perhaps two origins on the chromosome, giving rise to two linkage groups. All of the biosynthetic and antibiotic resistance genes that have been mapped, including those involved in photosynthesis, occur on one or another of these linkage groups.

Transformation of Rhodopseudomonas sphaeroides with deoxyribonucleic acid isolated from bacteriophage R phi 6P

The transformation of the photosynthetic bacterium Rhodopseudomonas sphaeroides with the circular genome of the penicillinase-encoding, temperate bacteriophage R phi 6P was demonstrated. The transformation was dependent on the infection of the recipient by another, apparently closely related, temperature bacteriophage, R phi 9. Optimum transformation occurred in the recipient cells already lysogenic for R phi 9 when superinfected with that bacteriophage at multiplicities of infection between 1 and 10 R phi 9 particles per recipient cell.