<p>Substitution rates (y-axis) at amino-acid positions (x-axis, numbers based on (A) the periplasmic region of <i>S</i>. <i>meliloti</i> ExoS and (B) full-length ExoR) were obtained using Rate4Site [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0135655#pone.0135655.ref094" target="_blank">94</a>]. While variable and conserved residues are evenly dispersed in ExoR (B), two variable residues are notable and specific to two regions in ExoS (A). We hypothesize, subject to experimental verification, that these two variable regions form binding domains to ExoR and are the molecular basis of host-specific responses among Rhizobiales species.</p
