Phylogenetic analysis of erythritol catabolic loci within the Rhizobiales and Proteobacteria

BACKGROUND: The ability to use erythritol as a sole carbon source is not universal among the Rhizobiaceae. Based on the relatedness to the catabolic genes in Brucella it has been suggested that the eryABCD operon may have been horizontally transferred into Rhizobium. During work characterizing a locus necessary for the transport and catabolism of erythritol, adonitol and L-arabitol in Sinorhizobium meliloti, we became interested in the differences between the erythritol loci of S. meliloti and R. leguminosarum. Utilizing the Ortholog Neighborhood Viewer from the DOE Joint Genome Institute database it appeared that loci for erythritol and polyol utilization had distinct arrangements that suggested these loci may have undergone genetic rearrangements. RESULTS: A data set was established of genetic loci containing erythritol/polyol orthologs for 19 different proteobacterial species. These loci were analyzed for genetic content and arrangement of genes associated with erythritol, adonitol and L-arabitol catabolism. Phylogenetic trees were constructed for core erythritol catabolic genes and contrasted with the species phylogeny. Additionally, phylogenetic trees were constructed for genes that showed differences in arrangement among the putative erythritol loci in these species. CONCLUSIONS: Three distinct erythritol/polyol loci arrangements have been identified that reflect metabolic need or specialization. Comparison of the phylogenetic trees of core erythritol catabolic genes with species phylogeny provides evidence that is consistent with these loci having been horizontally transferred from the alpha-proteobacteria into both the beta and gamma-proteobacteria. ABC transporters within these loci adopt 2 unique genetic arrangements, and although biological data suggests they are functional erythritol transporters, phylogenetic analysis suggests they may not be orthologs and probably should be considered analogs. Finally, evidence for the presence of paralogs, and xenologs of erythritol catabolic genes in some of the genomes included in the analysis is provided.