Abstract
Sterols are key components of eukaryotic cellular membranes that are synthesized by multi-enzyme pathways that require molecular oxygen. Because prokaryotes fundamentally lack sterols, it is unclear how the vast diversity of bacterivorous eukaryotes that inhabit hypoxic environments obtain, or synthesize, sterols. Here we show that tetrahymanol, a triterpenoid that does not require molecular oxygen for its biosynthesis, likely functions as a surrogate of sterol in eukaryotes inhabiting oxygen-poor environments. Genes encoding the tetrahymanol synthesizing enzyme squalene-tetrahymanol cyclase were found from several phylogenetically diverged eukaryotes that live in oxygen-poor environments and appear to have been laterally transferred among such eukaryotes.ReviewersThis article was reviewed by Eric Bapteste and Eugene Koonin.
Highlights
Sterols are key components of eukaryotic cellular membranes that are synthesized by multi-enzyme pathways that require molecular oxygen
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]
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 palustris, and the nitrogen-fixing bacterium Bradyrhizobium japonicum, in O
Summary
Sterols are key components of eukaryotic cellular membranes that are synthesized by multi-enzyme pathways that require molecular oxygen. 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.
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