Micronucleus-specific bacterium Holospora elegans irreversibly enhances stress gene expression of the host Paramecium caudatum.

The bacterium Holospora is an endonuclear symbiont of the ciliate Paramecium. Previously, we reported that paramecia bearing the macronuclear-specific symbiont Holospora obtusa survived better than symbiont-free paramecia, even under high temperatures unsuitable for growth. The paramecia with symbionts expressed high levels of hsp70 mRNAs even at 25 degrees C, a usual growth temperature. We report herein that paramecia bearing the micronuclear-specific symbiont Holospora elegans also acquire the heat-shock resistance. Even after the removal of the bacteria from the hosts by treatment with penicillin, the resulting aposymbiotic paramecia nevertheless maintained their heat shock-resistant nature for over 1 yr. Like symbiotic paramecia, these aposymbiotic paramecia also expressed high levels of both hsp60 and hsp70 mRNAs even at 25 degrees C. Moreover, analysis by fluorescent in situ hybridization with a probe specific for Holospora 16S rRNA revealed that the 16S rRNA of H. elegans was expressed around the nucleoli of the macronucleus in the aposymbiotic cells. This result suggests the possible transfer of Holospora genomic DNA from the micronucleus into the macronucleus in symbiotic paramecia. Perhaps this exogenous DNA could trigger the aposymbiotic paramecia to induce a stress response, inducing higher expression of Hsp60 and Hsp70, and thus conferring heat-shock resistance.