Fate of the 63-kDa periplasmic protein of the infectious form of the endonuclear symbiotic bacterium Holospora obtusa during the infection process.

Holospora obtusa is a macronucleus-specific endosymbiotic bacterium of the ciliate Paramecium caudatum. We report the secretion of a 63-kDa periplasmic protein of an infectious form of the bacterium into the macronucleus of its host. Indirect immunofluorescence microscopy with five monoclonal antibodies against the 63-kDa protein demonstrated that, soon after the bacterial invasion into the host macronucleus, the protein was detected in the infected macronucleus and that levels of the protein increased dramatically within one day of infection. The use of inhibitors for host and bacterial protein synthesis illustrated that, in early infection of H. obtusa, not only the pre-existing but also a newly synthesized 63-kDa protein was secreted into the host macronucleus. A partial amino acid sequence of the protein was determined, and a gene encoding the 63-kDa protein was cloned. The deduced amino acid sequence shows that this protein is a novel protein.