THE MOLECULAR-BIOLOGY OF A BACTERIAL ENDOSYMBIONT

The marine ciliate Parauronema acutum harbors a group of symbiotic bacteria, termed xenosomes, which reside exclusively in the cytoplasm where they grow and divide in a remarkable synchronism with the host. When released into the ambient culture medium by gentle rupture of the protozoans, the symbionts can infect homologous as well as heterologous Parauronema stocks and, oddly, Miamiensis avidus, a distantly related marine ciliate. Xenosomes from certain Parauronema stocks can kill other marine ciliates, particularly those of the genus Uronema. Our principal aims with this host-symbiont system have been to study, at the molecular level, the nature of the interaction of the xenosome with the host, infection and the killer effect. Our most recent investigations have been directed toward establishing the phylogenetic origins of the symbionts using molecular approaches. This paper summarizes our previous work and updates our more recent studies on the association of a bacterial symbiont with its protozoan host. The reader is referred to previous reviews for more details on the subject [4, 5].