The extracellular proteome of Rhizobium etli CE3 in exponential and stationary growth phase

The extracellular proteins were obtained by phenol extraction and identified by LC-ESI MS/MS. We identified 192 and 191 proteins for the exponential and stationary phases respectively. Using the software Signal P, we predicted signal peptides for 12.95% and 35.60% of the proteins identified in the exponential and stationary phases, respectively, which could therefore be secreted by the Sec pathway. For the exponential growth phase, we found in abundance proteins like the ribosomal proteins, toxins and proteins belonging to the group "defence mechanisms". For the stationary growth phase, we found that the most abundant proteins were those with unknown function, and in many of these we identified characteristic domains of proteases and peptidases.Our study provided the first dataset of the secretome of R. etli and its modifications, which may lead to novel insights into the adaptive response of different stages of growth. In addition, we found a high number of proteins with unknown function; these proteins could be analyzed in future research to elucidate their role in the extracellular proteome of R. etli.Nitrogen-fixing symbioses between plants belonging to the family Leguminosae and soil bacteria collectively called rhizobia, contribute substantially to plant productivity. Rhizobium etli is a Gram-negative soil bacterium that carries out symbiosis with bean plants, specifically with Phaseolus vulgaris. The plant offers the bacteria a source of carbon (product of photosynthesis) in exchange for fixed nitrogen. This association allows legumes, like bean, to grow in nitrogen poor soil. To establish symbiosis the two partners exchange a series of molecular signals. In general, symbiotic or pathogenic processes are mediated by elicitors that are perceived by receptors on the cell membrane of the plant. These elicitors may be directly synthesized by bacteria and included in cell wall components, like lipopolysaccharides, proteins or peptides, or they may be low molecular