Elevated levels of stress proteins associated with bacterial symbiosis in Amoeba proteus and soybean root nodule cells.

Obligatory bacterial endosymbionts of Amoeba proteus and symbiotic Bradyrhizobium japonicum bacteroids in soybean-root nodules contained large amounts of 67-kDa and 65-kDa proteins, respectively, antigenically related to groEL of E. coli and the 58-kDa heat-shock protein of Tetrahymena. Monoclonal antibodies against the 67-kDa protein recognized groEL analogs from several different organisms. The quantity of the stress protein in symbiotic B. japonicum bacteroids was augmented seven times that in the free-living counterparts. The increase in these proteins in endosymbionts, as determined by immunoblot techniques, indicated that intracellular symbiosis is a stress condition even when the symbiotic relationship is considered to be mutually beneficial. Mitochondria and chloroplasts may also be under a stressed condition like endosymbionts in view of the presence of heat-shock proteins in these cell organelles.