FIG 2.

S. meliloti lacks a PopA ortholog. C. crescentus PopA was used to perform a reciprocal blastp search for orthologs. Sequences with a minimum of 21% identity were used to create a maximum likelihood phylogeny tree. If more than one ortholog was found in the same organism, the sequences were randomly labeled 1 and 2. C. crescentus PleD was included in the final sequence alignment and phylogenetic analysis for comparison. PopA is present in Caulobacterales and Rhodobacterales but absent from many alphaproteobacteria that engage in host-microbe interaction, including Bradyrhizobiaceae, Brucellaceae, and most members of the Rhizobiaceae family within the Rhizobiales order, as well as the Rickettsiales. Several species have a PopA ortholog that is more homologous to those present in a different order, and two PopA orthologs were identified outside the alphaproteobacterial division and are indicated by asterisks as follows. Woodsholea maritima (*) is a member of the Caulobacterales. Scytonema millei (**) and Mastigocoleus testarum (***) are from the division Cyanobacterium. Polymorphum gilvum (****) is an unclassified alphaproteobacterium with a PopA ortholog most closely related to those of the Rhodobacterales.