Conservation of the Essential Genome Among Caulobacter and Brevundimonas Species

When the genomes of Caulobacter isolates NA1000 and K31 were compared, numerous genome rearrangements were observed. In contrast, similar comparisons of closely related species of other bacterial genera revealed nominal rearrangements. A phylogenetic analysis of the 16S rRNA indicated that K31 is more closely related to Caulobacter henricii CB4 than to other known Caulobacters. Therefore, we sequenced the CB4 genome and compared it to all of the available Caulobacter genomes to study genome rearrangements, discern the conservation of the NA1000 essential genome, and address concerns about using 16S rRNA to group Caulobacter species. We also sequenced the novel bacteria, Brevundimonas DS20, a representative of the genus most closely related to Caulobacter and used it as part of an outgroup for phylogenetic comparisons. We expected to find that there would be fewer rearrangements when comparing more closely related Caulobacters. However, we found that relatedness was not correlated with the amount of observed "genome scrambling." We also discovered that nearly all of the essential genes previously identified for C. crescentus are present in the other Caulobacter genomes and in the Brevundimonas genomes as well. However, a few of these essential genes were only found in NA1000, and some were missing in a combination of one or more species, while other proteins were 100 % identical across species. Also, phylogenetic comparisons of highly conserved genomic regions revealed clades similar to those identified by 16S rRNA-based phylogenies, verifying that 16S rRNA sequence comparisons are a valid method for grouping Caulobacters.