SUMMARY
Denitrification plays a central role in the global nitrogen cycle,
reducing and removing nitrogen from marine and terrestrial ecosystems.
The flux of nitrogen species through this pathway has widespread impact,
affecting ecological carrying capacity, agriculture, and climate.
Nitrite reductase (Nir) and nitric oxide reductase (NOR) are the two
central enzymes in this pathway. Here we present a previously unreported
Nir domain architecture in members of Phylum Chloroflexi. Phylogenetic
analyses of protein domains within Nir indicate that an ancestral
horizontal transfer and fusion event produced this chimeric domain
architecture. We also identify an expanded genomic diversity of a rarely
reported nitric oxide reductase subtype, eNOR. Together, these results
suggest a greater diversity of denitrification enzyme arrangements exist
than have been previously reported.