Global protein expression pattern of Bradyrhizobium japonicum bacteroids: a prelude to functional proteomics

As a prelude to using functional proteomics towards understanding the process of symbiotic nitrogen fixation between the legume soybean and the soil bacteria Bradyrhizobium japonicum, we examined the total protein expression pattern of the nodule bacteria, often referred to as bacteroids. A partial proteome map was constructed by separating the total bacteroid proteins using high-resolution 2-DE. Of the several hundred protein spots analyzed using PMF, 180 spots were tentatively identified by searching the available database for B. japonicum, (http://www.kazusa.or.jp/index.html). The data showed that the bacteroid expressed a dominant and elaborate protein network for nitrogen and carbon metabolism, which is closely dependent on the plant supplied metabolites, and seems aptly supported by a selective group of bacteroid transporter proteins. However, they seem to lack a defined fatty acid and nucleic acid metabolism. Interestingly, the proteins related to protein synthesis, scaffolding and degradation were among the most predominant spots of the bacteroid proteome. In addition, several proteins, which showed fairly good expression, were identified to be involved with cellular detoxification, stress regulation and signaling communication components. This preliminary proteomic data matches very well with several biochemical and genetic reports, and clearly shows the inter-connection between several metabolic pathways that meet the needs of the bacteroid. It is expected that in the future this will allow us to develop testable hypotheses about the roles of several of these proteins in context to the metabolic pathway connections and metabolite fluxes.