Identification of "nodule-specific" host proteins (nodoulins) involved in the development of rhizobium-legume symbiosis.

Infection of legume roots with Rhizobium species results in the development of a root nodule structure in which the bacteria form an intracellular symbiosis with the plant. We report here that the infection of soybean (Glycine max L.) roots with Rhizobium japonicum results in the synthesis by the plant of at least 18-20 polypeptides other than leghemoglobin during the development of root nodules. Identification of these "nodule-specific" host polypeptides (referred to as nodulins) was accomplished by two-dimensional gel analysis of the immunoprecipitates formed by a "nodule-specific" antiserum with in vitro translation products of root-nodule polysomes that are free of bacteroidal contaminations. Nodulins account for 7-11% of the total 35S-methionine-labeled protein synthesized in the host cell cytoplasm, and the majority of them are of 12,000-20,000 molecular weight. These proteins are absent from the uninfected roots, bacteroids and free-living Rhizobium, and appear to be coded for the plant genes that may be obligatory for the development of symbiosis in the legume root nodules. Analysis of nodulins in ineffective (unable to fix nitrogen) nodules developed due to Rhizobium strains SM5 and 61A24 showed that their synthesis is reduced and their expression differentially influenced by mutations in rhizobia. Two polypeptides of bacterial origin were also found to be cross-reactive with the "nodule-specific" antiserum, suggesting that they are secreted by Rhizobium into the host cell cytoplasm during symbiotic nitrogen fixation.