Lateral transfer of tetrahymanol-synthesizing genes has allowed multiple diverse eukaryote lineages to independently adapt to environments without oxygen

This article was reviewed by Eric Bapteste and Eugene Koonin.A large fraction of eukaryotes and bacteria possess sterols and hopanoids respectively that function as potent stabilizers of cell membranes. Sterols are also associated with fluidity and permeability of eukaryotic cell membranes, and are key to fundamental eukaryotic-specific cellular processes such as phagocytosis [e.g. [1,2]]. Several steps of de novo sterol biosynthesis require molecular oxygen [3]. For example, the epoxidation of squalene is the first oxygen-dependent step in the sterol pathway; the epoxidized squalene is then cyclized to either lanosterol or cycloartenol by the enzyme oxidosqualene cyclase (OSC). In contrast, prokaryotic hopanoid biosynthesis does not require molecular oxygen as a substrate, and the squalene is directly cyclized by the enzyme squalene-hopene cyclase (SHC) [4].Until now, it was unclear how bacterivorous unicellular eukaryotes that are abundant in anoxic or low oxygen environments can carry out phagocytosis. These eukaryotes cannot obtain sterols from food bacteria as the latter generally lack them and sterols cannot be synthesized de novo in the absence of molecular oxygen. Explanations that seem plausible are: 1) anaerobic eukaryotes could acquire free sterols from the environment, or 2) they could anaerobically synthesize sterol-like molecules de novo using alternative biochemical pathways. Here we provide evidence for the latter by showing that the molecule tetrahymanol is synthesized by anaerobic/microaerophilic eukaryotes and functions as an analogue of sterols in these organisms.Tetrahymanol is a triterpenoid with five cyclohexyl rings that does not require molecular oxygen for its synthesis. It was first discovered in the ciliated protozoan Tetrahymena pyriformis [5] but has been more recently detected in other ciliates, the anaerobic rumen fungus Piromonas (Piromyces) communis, the fern Oleandra wallichii, the purple nonsulfur bacterium Rhodopseudomonas palust