Abstract
Anaeromyxobacter dehalogenans strain 2CP-C is a versaphilic delta-Proteobacterium distributed throughout many diverse soil and sediment environments. 16S rRNA gene phylogenetic analysis groups A. dehalogenans together with the myxobacteria, which have distinguishing characteristics including strictly aerobic metabolism, sporulation, fruiting body formation, and surface motility. Analysis of the 5.01 Mb strain 2CP-C genome substantiated that this organism is a myxobacterium but shares genotypic traits with the anaerobic majority of the delta-Proteobacteria (i.e., the Desulfuromonadales). Reflective of its respiratory versatility, strain 2CP-C possesses 68 genes coding for putative c-type cytochromes, including one gene with 40 heme binding motifs. Consistent with its relatedness to the myxobacteria, surface motility was observed in strain 2CP-C and multiple types of motility genes are present, including 28 genes for gliding, adventurous (A-) motility and 17 genes for type IV pilus-based motility (i.e., social (S-) motility) that all have homologs in Myxococcus xanthus. Although A. dehalogenans shares many metabolic traits with the anaerobic majority of the delta-Proteobacteria, strain 2CP-C grows under microaerophilic conditions and possesses detoxification systems for reactive oxygen species. Accordingly, two gene clusters coding for NADH dehydrogenase subunits and two cytochrome oxidase gene clusters in strain 2CP-C are similar to those in M. xanthus. Remarkably, strain 2CP-C possesses a third NADH dehydrogenase gene cluster and a cytochrome cbb 3 oxidase gene cluster, apparently acquired through ancient horizontal gene transfer from a strictly anaerobic green sulfur bacterium. The mosaic nature of the A. dehalogenans strain 2CP-C genome suggests that the metabolically versatile, anaerobic members of the delta-Proteobacteria may have descended from aerobic ancestors with complex lifestyles.
Highlights
Classification of the eubacterial domain remains a major challenge in prokaryotic taxonomy
The analysis provides evidence for ancient horizontal gene transfer from another bacterial domain and supports the hypothesis that respiratory versatility in A. dehalogenans is a derived trait, one that was gained after splitting from an aerobic ancestor that is common to the myxobacteria and possibly the entire delta-Proteobacteria class
According to 16S rRNA gene phylogeny, A. dehalogenans is a delta-Proteobacterium that is deeply nested in the order Myxococcales (Figure 1)
Summary
Classification of the eubacterial domain remains a major challenge in prokaryotic taxonomy. 16S rRNA gene phylogeny is unreliable for predicting physiology but this analysis does typically provide information about an organism’s evolutionary history [3,4]. When applied to genomic analyses, phylogeny deduced from the 16S rRNA gene sequence provides a framework for using genomic information to interpret evolution by distinguishing derived traits from those of a common ancestor. Anaeromyxobacter dehalogenans strains were initially isolated from pristine soils based on their ability to derive energy from reductive dechlorination of chlorophenols [5,6]. Characteristic for A. dehalogenans strains is great respiratory versatility including metal and radionuclide reduction and recent efforts have yielded additional isolates from contaminated subsurface environments and agricultural soils [7,8,9,10]. Anaeromyxobacter spp. are the first anaerobes that group with the order Myxococcales (traditionally called ‘myxobacteria’) according to 16S rRNA gene phylogeny
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