The regulator purR evolved by duplication from the ribose repressor rbsR, itself acquired by HGT. Within the Enterobacteria/Vibrionaceae subgroup of the γ-Proteobacteria, both rbsR and purR exhibit largely vertical evolution. The closest relatives of rbsR and purR from outside this subgroup of γ-Proteobacteria are associated with genes for ribose utilization and probably function as ribose repressors. The absence of both rbsR and purR from Buchnera and its relatives and from Sodalis might suggest additional transfer events, but because Buchnera and its relatives have under 700 genes, absence from this clade is not evidence for horizontal gene transfer (HGT). Sodalis is also a reduced genome, with around 2,600 genes, whereas most Enterobacteria have over 4,000 genes. The purR/rbsR tree was computed from protein sequences with phyml and 100 bootstraps (as in Figure 3).