SUMMARY: Kappa, a bacterial endosymbiont present in killer stocks of , exists in two forms - brights, which contain a refractile (R) body, and non-brights which lack the R body. Non-brights are self-reproducing and give rise to brights, which cannot reproduce and are toxic to sensitive paramecia which ingest them. Associated with the R body are structures resembling bacteriophages in form and chemical composition. Animals of stock 51, syngen 4 of were given low doses of ultraviolet light. The percentage of brights increased markedly, beginning 6 h after irradiation. In three experiments, absolute numbers of kappas were determined 13 to 14 h after exposure to u.v.: the number of brights increased and non-brights decreased significantly, while the total number of kappas remained the same. At 24 h, after irradiation the numbers of kappa particles had decreased considerably; after 3 days they were completely gone. The results are consistent with the hypothesis that the phage-like structures exist as u.v.-inducible prophages in non-brights, and that when a non-bright becomes a bright, the prophage is spontaneously induced forming, at the same time, mature phage-like elements and the R body. These events constitute the lytic cycle which ends with the destruction of the bright.


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