Bacteria belonging to the genus Caedibacter are a genetically diverse group that have three traits in common. They are: 1.) obligate cytoplasmic endosymbionts of certain strains of Paramecium biaurelia and P. tetraurelia; 2.) capable of inducing lethal reactions in uninfected (sensitive) strains of paramecia upon ingestion; and 3.) capable of producing a large inclusion body known as an R body. The latter two traits are not expressed at all times. However, when one trait is expressed the other is expressed as well. The genetic determinants for both toxicity and R body production are probably extrachromosomal. In one species, C. taeniospiralis, these traits are most likely determined by a very closely related group of plasmids whereas in the other three species of Caedibacter these traits are apparently determined by bacteriophage DNA sequences. We have demonstrated that a 2600 base pair DNA sequence derived from a 49kb plasmid carried by C. taeniospiralis strain 47 will direct R body synthesis in various strains of Escherichia coli. The specific objectives that we expect to attain during the tenure of this proposal are: 1.) to determine the nucleotide sequence of the cloned genes that direct R body synthesis in E. coli; 2.) to determine what portions of the cloned sequence code for protein products and how they are regulated; 3.) to characterize the protein products of this cloned sequence; 4.) to determine the proceses involved in R body assembly; 5.) to determine the relationships among the DNA sequences that code for R body production in the various strains and species of Caedibacter; 6.) to determine the relationships between R body proteins, bacteriophage proteins and the toxic activity against sensitive paramecia; and 7.) to study the relationships of the variable sequences that have been observed in the plasmids of C. taeniospiralis.