The Size and Structure of the DNA Genome of Symbiont Xenosome Particles in the Ciliate Parauronema acutum

Abstract

The size and structure of the DNA genome of xenosomes, bacterial endosymbionts of the marine hymenostome ciliate, P. acutum 110-3, were investigated. Renaturation kinetic measurements, determined optically and by hydroxylapatite chromatography, suggested a genome size of 0.34 .times. 109 daltons. Sedimentation rate measurements of DNA gently released from the symbionts yielded molecules of comparable size. The analytical complexity, determined chemically, was 3.03 .times. 109 daltons. Consistent with these and other data is a model for the structure of the symbiont genome in which the DNA exists in the form of 9 circularly permuted, double-stranded DNA molecules of unique sequence, each of MW 0.34 .times. 109. Xenosomes and certain symbionts found in ciliated protozoa may be extant forms of once free-living bacteria that adapted to the intracellular environment.