Catechol 2,3-dioxygenase and other meta-cleavage catabolic pathway genes in the "anaerobic" termite gut spirochete Treponema primitia
Microorganisms have evolved a spectacular diversity of metabolisms, some of which allow them to overcome environmental constraints, utilize abundant but inaccessible resources, and drive nutrient cycling in various ecosystems. The termite hindgut microbial community is optimized to metabolize wood and in recent years, the in situ physiological and ecological functions of community members have been researched. Spirochetes are abundant in the termite gut and herein, putative aromatic meta-cleavage pathway genes typical of aerobic pseudomonads were located in genomes of homoacetogenic termite hindgut "anaerobes," Treponema primitia str. ZAS-1 and ZAS-2. Phylogenetic analyses suggest the T. primitia catechol 2,3-dioxygenase and several other essential meta-pathway genes were acquired from an α-proteobacterium in the distant past to augment several genes T. primitia acquired from anaerobic firmicutes that do not directly catabolize aromatics but can contribute to the final pathway steps. Further, transcripts for each meta-pathway gene were expressed in strictly anaerobic cultures of T. primitia str. ZAS-2 indicative of constitutive pathway expression. Also, the addition of catechol + O_2 to T. primitia liquid cultures resulted in the transient accumulation of trace amounts of the yellow ring cleavage product, hydroxymuconic semialdehyde. This is the first evidence of aromatic ring cleavage in the phylum (division) Spirochetes. Results also support a possible role for T. primitia in termite hindgut O_2/lignin aromatic monomer metabolism. Potential O_2-dependent yet non-respiratory microbial metabolisms have heretofore been overlooked and warrant further investigation. These metabolisms could describe the degradation of plant-derived and other aromatics in microoxic environments, and contribute significantly to carbon turnover.
© 2013 John Wiley & Sons Ltd. Received May 13, 2013; Revised Nov. 17, 2013; Accepted Nov. 18, 2013. This research was supported by NSF Graduate Fellowship (K. L.) and by grants from the DOE (DE-FG02-07ER64484), the NSF (EFRI-1137249), and the Center for Environmental Microbial Interactions (CEMI) at Caltech. We thanks A. Z. Rosenthal and our laboratory colleagues for comments on early versions of this manuscript. Author Contributions: K.S.L. and J.R.L. conceived and designed the experiments, analyzed the data, and wrote and edited the manuscript. K.S.L. carried out the experiments. Protein sequence GenBank accessions CP001843 (Treponema primitia str. ZAS-1 and ZAS-2), CP001841 Treponema azotonutricium str. ZAS-9) and ABDH00000000 (P3 hindgut region from Nasutitermes sp). More extensive accession information is provided in Table S1 (Supporting information). Raw data including growth optical density, depth of O_2 penetration in Treponema primitia O_2/aromatic gradient cultures, as well as original MUS-CLE alignments, ARB outputs and MrBayes outputs for phylogenetic trees. Dryad doi: 10.5061/dryad.5248s.
Supplemental Material - mec12598-sup-0001-TableS1-S4-FigS1-S18.pdf