Genes involved in Sinorhizobium meliloti's exopolysaccharide synthesis pathway and symbiosis

The alphaproteobacterium Sinorhizobium meliloti colonizes the roots of host plants, such as Medicago species, and induces the formation of nodules, where it fixes molecular nitrogen in exchange for carbon compounds. A previous suppressor analysis had identified two genes, SMc00067 and SMc03872, involved in the synthesis of exopolysaccharide-I (EPS-I), critical for symbiosis. A loss-of-function mutation in either gene resulted in lower production of EPS-I. We used epistasis analysis to investigate the relationship between SMc00067 and SMc03872 and other EPS-I synthesis genes (exoY, exoS, exoR and, expR+). I confirmed lower EPS-I levels in the ASMc00067 and ASMc03872 mutants by direct measurement via anthrone assays. I also found reduced expression of exoY, encoding an enzyme required for EPS-I synthesis, in the mutants. We expected the EPS-I under-production to affect the ability of these two mutant strains to infect and form nodules on alfalfa roots. ASMc00067 and ASMc03872 mutants, although defective in EPS-I production, did not show any defects in stimulating nodules compared to the wild type strain. Further study will uncover the roles of these genes and the regulatory pathways that influences EPS-I production and symbiosis.