Formate dehydrogenase gene diversity in lignocellulose - feeding insect gut microbial communities

The bacterial Wood-Ljungdahl pathway for CO2-reductive acetogenesis is important for the nutritional mutualism occurring between wood-feeding insects and their hindgut microbiota. A key step in this pathway is the reduction of CO2 to formate, catalyzed by the enzyme formate dehydrogenase (FDH). Putative selenocysteine(Sec) and cysteine(Cys) containing paralogs of hydrogenase-linked FDH (FDHH) have been identified in the termite gut acetogenic spirochete, Treponema primitia, but knowledge of their relevance in the termite gut environment remains limited. In this study, we designed degenerate PCR primers for FDHH genes (fdhF) and assessed fdhF diversity in insect gut bacterial isolates and the gut microbial communities of termites and roaches. The insects examined herein represent the wood-feeding termite families Termopsidae, Kalotermitidae, and Rhinotermitidae (phylogenetically “lower” termite taxa), the wood-feeding roach family Cryptocercidae (the sister taxon to termites), and the omnivorous roach family Blattidae. Sec and Cys FDHH variants were identified in every wood-feeding insect but not the omnivorous roach. Of 68 novel phylotypes obtained from inventories, 66 affiliated phylogenetically with enzymes from T. primitia. These formed two sub-clades (37 and 29 phylotypes) almost completely comprised of Sec-containing and Cys-containing enzymes, respectively. A gut cDNA