This model represents a family of TonB-dependent outer-membrane receptors which are found mainly in Xanthomonas and Caulobacter. These appear to represent the expansion of a paralogous family in that the 22 X. axonopodis (21 in X. campestris) and 18 C. crescentus sequences are more closely related to each other than any of the many TonB-dependent receptors found in other species. In fact, the Crescentus and Xanthomonas sequences are inseparable on a phylogenetic tree using a PAM-weighted neighbor-joining method, indicating that one of the two genuses may have acquired this set of receptors from the other. The mechanism by which this family is shared between Xanthomonas, a gamma proteobacterial plant pathogen and Caulobacter, an alpha proteobacterial aquatic organism is unclear. [Transport and binding proteins, Porins]
Jiyao W et al.(2023). "The conserved domain database in 2023.",