Specialization in a Nitrogen-Fixing Symbiosis: Proteome Differences Between Sinorhizobium medicae Bacteria and Bacteroids

Using tandem mass spectrometry (MS/MS), we analyzed the proteome of Sinorhizobium medicae WSM419 growing as free-living cells and in symbiosis with Medicago truncatula. In all, 3,215 proteins were identified, over half of the open read-ing frames predicted from the genomic sequence. The abun-dance of 1,361 proteins displayed strong lifestyle bias. In total, 1,131 proteins had similar levels in bacteroids and free-living cells, and the low levels of 723 proteins prevented statistically significant assignments. Nitrogenase subunits comprised approximately 12% of quantified bacteroid pro-teins. Other major bacteroid proteins included symbiosis-specific cytochromes and FixABCX, which transfer electrons to nitrogenase. Bacteroids had normal levels of proteins involved in amino acid biosynthesis, glycolysis or gluconeo-genesis, and the pentose phosphate pathway; however, sev-eral amino acid degradation pathways were repressed. This suggests that bacteroids maintain a relatively independent anabolic metabolism. Tricarboxylic acid cycle proteins were highly expressed in bacteroids and no other catabolic path-way emerged as an obvious candidate to supply energy and reductant to nitrogen fixation. Bacterial stress response pro-teins were induced in bacteroids. Many WSM419 proteins that are not encoded in S. meliloti Rm1021 were detected, and understanding the functions of these proteins might clar-ify why S. medicae WSM419 forms a more effective symbiosis with M. truncatula than S. meliloti Rm1021.

Automated summary

The proteome of Sinorhizobium medicae WSM419 was analyzed using tandem mass spectrometry, revealing lifestyle bias in the abundance of proteins. Nitrogenase subunits comprised approximately 12% of quantified bacteroid proteins, indicating the importance of nitrogen fixation in symbiosis with Medicago truncatula. Bacterial stress response proteins were induced in bacteroids, suggesting an adaptation to the symbiotic environment.