First insights into the syntrophic acetate‐oxidizing bacteria – a genetic study

<jats:title>Abstract</jats:title><jats:p>Syntrophic acetate‐oxidizing bacteria have been identified as key organisms for efficient biogas production from protein‐rich materials. They normally grow as lithotrophs or heterotrophs, producing acetate through the <jats:styled-content style="fixed-case">W</jats:styled-content>ood–<jats:styled-content style="fixed-case">L</jats:styled-content>jungdahl pathway, but when growing in syntrophy with methanogens, they reportedly reverse this pathway and oxidize acetate to hydrogen and carbon dioxide. However, the biochemical and regulatory mechanisms behind the shift and the way in which the bacteria regain energy remain unknown. In a genome‐walking approach, starting with degenerated primers, we identified those gene clusters in <jats:italic><jats:styled-content style="fixed-case">S</jats:styled-content>yntrophaceticus schinkii</jats:italic>,<jats:italic> <jats:styled-content style="fixed-case">C</jats:styled-content>lostridium ultunense</jats:italic>, and <jats:italic><jats:styled-content style="fixed-case">T</jats:styled-content>epidanaerobacter acetatoxydans</jats:italic> that comprise the formyltetrahydrofolate synthetase gene (<jats:styled-content style="fixed-case"><jats:italic>fhs</jats:italic></jats:styled-content>), encoding a key enzyme of the <jats:styled-content style="fixed-case">W</jats:styled-content>ood–<jats:styled-content style="fixed-case">L</jats:styled-content>jungdahl pathway. We also discovered that the latter two harbor two <jats:styled-content style="fixed-case"><jats:italic>fhs</jats:italic></jats:styled-content> alleles. The <jats:styled-content style="fixed-case"><jats:italic>fhs</jats:italic></jats:styled-content> genes are phylogenetically separated and in the case of <jats:italic><jats:styled-content style="fixed-case">S</jats:styled-content>. schinkii</jats:italic> functionally linked to sulfate reducers. The <jats:italic><jats:styled-content style="fixed-case">T</jats:styled-content>. acetatoxydans </jats:italic><jats:styled-content style="fixed-case"><jats:italic>fhs</jats:italic></jats:styled-content>1 cluster combines features of acetogens, sulfate reducers, and carbon monoxide oxidizers and is organized as a putative operon. The <jats:italic><jats:styled-content style="fixed-case">T</jats:styled-content>. acetatoxydans </jats:italic><jats:styled-content style="fixed-case"><jats:italic>fhs</jats:italic></jats:styled-content>2 cluster encodes <jats:styled-content style="fixed-case">W</jats:styled-content>ood–<jats:styled-content style="fixed-case">L</jats:styled-content>jungdahl pathway enzymes, which are also known to be involved in C1 carbon metabolism. Isolation of the enzymes illustrated that both formyltetrahydrofolate synthetases of <jats:italic><jats:styled-content style="fixed-case">T</jats:styled-content>. acetatoxydans</jats:italic> were functionally active. However, only <jats:styled-content style="fixed-case"><jats:italic>fhs</jats:italic></jats:styled-content>1 was expressed, confirming bidirectional usage of the pathway.</jats:p>