Association of transformation of xenosomes from nonkiller to killer with extrachromosomal DNA

Abstract

Extrachromosomal DNA in the form of covalently closed circular DNA molecules was isolated from killer and nonkiller xenosomes, bacterial endosymbionts of the marine protozoan Parauronema acutum. Restriction endonuclease digests of these molecules derived from 12 isolates revealed consistent, readily identifiable, differences in the pattern of fragments of the killer as compared with those present in the nonkiller. Transformation of the nonkiller to killer by infection is also accompanied by a change from the nonkiller to killer pattern. Based on analysis of fragments resulting from restriction endonuclease digests, two circular duplex DNA molecules, each 63 kilobase pairs (kbp) in length, were identified in the 263-20 nonkiller stock and mapped. The maps revealed that each possesses a single BamHI site and multiple Bgll, BstIIE, PstI, and Sall sites. A distinguishing feature of these maps is that the two molecules share a region about 17 kbp in length in which multiple restriction sites are in register with each other. Allowing for a 0.5-kbp insertion or deletion and the introduction or removal of only a few restriction sites, an additional stretch extending approximately 31 kbp beyond this sequence could also be considered to be homologous. The structure of the killer plasmid appears to be more complex, and we have been unable, as yet, to construct physical maps for this DNA. We postulate that the killer plasmid DNA is composed of three, perhaps four, circular 63-kbp duplexes, at least one which contains a single BamHI site and another which contains two BamHI sites. The remaining molecules may represent copies of either or both of the other two, modified to contain additional restriction sites. Transformation from the nonkiller to the killer is visualized as the insertion of restriction sites at various points along parent nonkiller plasmid DNA molecules. The mechanism by which these sites are introduced is unknown.Several stocks of the marine protozoan Paraiuronema acutum contain intracytoplasmic symbiotic bacteria which grow and divide in synchronism with the host (13). The symbionts, which we have termed xenosomes, sensu strictu, are obligate parasites and cannot be cultured outside the protozoan. Xenosomes released from the cytoplasm by mechanical rupture of the host cell are capable of infecting homologous and heterologous strains of P. acutuem as well as one other marine protozoan, Miamiensis aliidus Ma. Xenosomes from some stocks (killers) are capable of killing certain marine protozoa of the genus Uronema as evidenced by exposure of these organisms to breis of symbiont-bearing P. acutum. We show here that xenosomes from other stocks of P. acutum (nonkillers) do not have this capacity. Significantly, P. acutum is insensitive to the toxin produced by the killer xenosomes. In an effort to determine the mechanism responsible for the killer effect and to establish its molecular basis, we previously examined chromosomal DNA derived from killer and nonkiller xenosomes by DNA-DNA hybridization (9). Whereas a h...

Neutral Lipids, their Fatty Acids, and the Sterols of the Marine Ciliated Protozoon, Parauronema acutum

“…While several of these marine ciliates, particularly Uronema , have been studied by electron microscopy (Kaneshiro and Holz 1976) and some physiological studies have been carried out on Miamiensis avidus and Uronema (Kaneshiro, Dunham, and Holz 1969; Thomson and Kaneshiro 1968), most studies have utilized P. acutum. These studies have focused on the nutrition, physiology, and xenosomes of P. acutum (Soldo 1978, 1987; Soldo et al 1986; Soldo, Brickson, and Larin 1981, 1983; Soldo, Godoy, and Larin 1978). Very little is known about the biochemistry of this ciliate.…”