Structure and Characterization of Extrachromosomal DNA of Killer Xenosomes, Intracellular Symbionts of a Marine Protozoan

Abstract

Xenosome is the term we have adopted to describe a group of infectious intracellular bacterial endosymbionts found in several strains of the marine ciliate Parauronema acutum. Whereas symbionts of each strain are capable of infecting homologous and heterologous protozoan stocks, xenosomes from only certain strains are capable of killing other marine protozoa, particularly those of the genus Uronema. Recently, we identified extrachromosomal DNA in the form of covalently closed circular duplexes, each 63 kb in length, from both killer and nonkiller xenosomes. Molecules from the killer exhibited consistent differences in gel patterns of fragments derived from restriction endonuclease digests as compared to the nonkiller. Moreover, transformation of nonkiller to killer by infection was accompanied by a change from the nonkiller to the killer pattern. A physical map of nonkiller plasmid DNA revealed the presence of two circular 63-kb du-plexes, each with a single BamHI site and a region of homology of about 17 kb in length. We report here that the killer plasmid DNA consists of four circular 63-kb duplexes, two of which are indistinguishable with respect to their restriction maps and possess two BamHI sites. Each of the remaining two contains only a single BamHI site. Like nonkiller plasmids, all these duplexes contain a region of about 17 kb in length in which all the restriction sites are in register. It is postulated that transformation from nonkiller to killer involves the insertion of additional restriction sites on two of the duplexes. We show also that under axenic conditions of growth, plasmid from both killer and nonkiller xenosomes is amplified during late stationary growth phase of the protozoa.