Structure of a functional archaellum in Bacteria of the Chloroflexota phylum.

Motility in Archaea is driven by the archaellum, a rotary ATP-driven machinery unrelated to the bacterial flagellum. To date, archaella have been described exclusively in archaea; however, recent work reported archaellum genes in bacterial strains of the SAR202 clade (Chloroflexota). Here, using MacSyFinder, we show that bona fide archaellum gene clusters are widespread in several members of the Chloroflexota. Analysis of archaellum-encoding loci and Alphafold3-predicted structures show similarity to the archaellum machinery. Using cryo electron microscopy single-particle analysis, we solved the structure of the bacterial archaellum from Litorilinea aerophila to 2.7 Å. We also show the expression and assembly of this machinery in bacteria and its function in swimming motility. Finally, a phylogenomic analysis revealed two horizontal gene transfer events from euryarchaeal members to Chloroflexota. In summary, our study shows that a functional and assembled archaellum machinery can be exchanged between the two prokaryotic domains.