Analysis of the secretome of the soybean symbiont Bradyrhizobium japonicum

Proteins from the supernatant of Bradyrhizobium japonicum were separated by two-dimensional gel electrophoresis and stained with Coomassie. This revealed more than 100 protein spots. Sixty-eight proteins were identified by mass spectrometry. Thirty-five are predicted to contain an N-terminal signal peptide characteristic for proteins transported by the general secretory pathway. Most of these appear to be substrate-binding proteins of the ABC transporter family. Ten proteins were categorized as unclassified conserved or hypothetical. None of the proteins has similarity to proteins transported by a type I secretion system or to autotransporters. Three of the proteins might be located in the outer membrane. The addition of genistein led to changes in the spot pattern of three flagellar proteins and resulted in the identification of the nodulation outer protein Pgl. Moreover, the application of shot-gun mass spectrometry resulted in the first-time identification of NopB, NopH and NopT, which were present only after genistein induction. Replacing genistein with daidzein or coumestrol reduced the amount of the type III-secreted protein GunA2.