Homology between the flagellar export apparatus and ATP synthetase: evidence from synteny predating the Last Universal Common Ancestor

Evidence of homology between proteins in the ATP synthetase and the bacterial flagellar motor (BFM) has been accumulating since the 1980s. Specifically, the BFM’s Type 3 Secretion System (T3SS) export apparatus FliH, FliI, and FliJ are considered homologous to F O -b + F 1 -δ, F 1 -α/β, and F 1 -γ, and have similar structure and interactions. We review the discoveries that advanced the homology hypothesis and then conduct a further test by examining gene order in the two systems and their relatives. Conservation of gene order, or synteny, is often observed between closely related prokaryote species, but usually degrades with phylogenetic distance. As a result, observed conservation of synteny over vast phylogenetic distances can be evidence of shared ancestral coexpression, interaction, and function. We constructed a gene order dataset by examining the order of fliH , fliI , and fliJ genes across the phylogenetic breadth of flagellar and nonflagellar T3SS. We compared this to published surveys of gene order in the F 1 F O -ATP synthetase, its N-ATPase relatives, and the bacterial/archaeal V- and A-type ATPases. Strikingly, the fliHIJ gene order was deeply conserved, with the few exceptions appearing derived, and exactly matching the widely conserved F-ATPase gene order atpFHAG , coding for subunits b - δ - α - γ . The V/A-type ATPases have a similar conserved gene order shared for homologous components. Our results further strengthen the argument for homology between these systems, and suggest a rare case of synteny conserved over billions of years, dating back to well before the Last Universal Common Ancestor (LUCA).