C.E. FLORES-TINOCO¹, F. TSCHAN¹, M. BARNETT², M. CHRISTEN¹, S. LONG², B. CHRISTEN<sup>1

1</sup>IMSB, ETH Zürich, CH-8093 Zürich, Switzerland, ²Stanford University, Stanford, CA 94305, USA

Sinorhizobium meliloti is an alpha-proteobacterium that can adopt a free-living lifestyle or engage in endo-symbiosis with legume plants. Associated with its distinct role as a nitrogen fixing endosymbiont of legumes, S. meliloti has evolved a remarkable adaptability to grow in nutritionally diverse environments. In the absence of its plant host, S. meliloti thrives as oligotrophic soil bacteria and hence must have evolved sophisticated strategies to adapt to diverse environmental conditions present within the rhizosphere. To identify the complete set of essential genes required for growth under variations in carbon source availability, we designed a systems-wide genetic screen using hyper-saturated Tn5 transposon mutagenesis in conjunction with next generation sequencing (TnSeq). We defined the essential genome of S. meliloti required for propagation under defined sets of nutrients. Across the 6.7Mb tripartite genome of S. meliloti, we mapped 750128 and 674886 unique Tn5 transposon insertion events under rich and minimal media growth. We identified 320 genes essential under both conditions. Surprisingly, we found more conditional genes for rich media (190 genes) than for minimal media (132 genes) growth. Under rich media conditions, a significant portion of 56 conditional essential genes (29.4%), participate in cell cycle regulation, while in minimal media 67 conditional essential genes (50.8%) participate in auxotrophic processes. In sum, our studies provide systems-level insights how the essential genome responds to variations in nutrient availability enabling persistence of S. meliloti in soil, and consequently improving its ability to colonize and survive within host plants.