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. 3215 proteins were identified, over half of the ORFs predicted from the genomic sequence. The abundance of 1361 proteins displayed strong lifestyle bias. 1131 proteins had similar levels in bacteroids and free-living cells, and the low levels of 723 proteins prevented statistically significant assignments. Nitrogenase subunits comprised ~12% of quantified bacteroid proteins. 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/gluconeogenesis and the pentose phosphate pathway, but several amino acid degradation pathways were repressed. This suggests bacteroids maintain a relatively independent anabolic metabolism. TCA cycle proteins were highly expressed in bacteroids and no other catabolic pathway emerged as an obvious candidate to supply energy and reductant to nitrogen fixation. Bacterial stress response proteins were induced in bacteroids. Many WSM419 proteins that are not encoded in Sinorhizobium meliloti Rm1021 were detected and understanding the functions of these proteins might clarify why S. medicae WSM419 forms a more effective symbiosis with M. truncatula than S. meliloti Rm1021.