A 63-kDa Periplasmic Protein of the Endonuclear Symbiotic Bacterium Secreted to the Outside of the Bacterium during the Early Infection Process Binds Weakly to the Macronuclear DNA of the Host .

The Gram-negative bacterium is a macronucleus-specific symbiont of the ciliate . It is known that an infection of this bacterium induces high level expressions of the host and genes, and the host cell acquires both heat-shock and high salt resistances. In addition, an infectious form of -specific 63-kDa periplasmic protein with a DNA-binding domain in its amino acid sequence is secreted into the host macronucleus after invasion into the macronucleus and remain within the nucleus. These facts suggest that binding of the 63-kDa protein to the host macronuclear DNA causes changes in the host gene expressions and enhances an environmental adaptability of the host cells. This 63-kDa protein was renamed as periplasmic region protein 1 (PRP1) to distinguish it from other proteins with similar molecular weights. To confirm whether PRP1 indeed binds to the host DNA, SDS-DNA PAGE and DNA affinity chromatography with calf thymus DNA and DNA were conducted and confirmed that PRP1 binds weakly to the DNA with a monoclonal antibody raised for the 63-kDa protein.