A squalene–hopene cyclase in <i>Schizosaccharomyces japonicus</i> represents a eukaryotic adaptation to sterol-limited anaerobic environments

Biosynthesis of sterols, which are key constituents of canonical eukaryotic membranes, requires molecular oxygen. Anaerobic protists and deep-branching anaerobic fungi are the only eukaryotes in which a mechanism for sterol-independent growth has been elucidated. In these organisms, tetrahymanol, formed through oxygen-independent cyclization of squalene by a squalene-tetrahymanol cyclase, acts as a sterol surrogate. This study confirms an early report [C. J. E. A. Bulder, <i>Antonie Van Leeuwenhoek</i>, 37, 353-358 (1971)] that <i>Schizosaccharomyces japonicus</i> is exceptional among yeasts in growing anaerobically on synthetic media lacking sterols and unsaturated fatty acids. Mass spectrometry of lipid fractions of anaerobically grown <i>Sch. japonicus</i> showed the presence of hopanoids, a class of cyclic triterpenoids not previously detected in yeasts, including hop-22(29)-ene, hop-17(21)-ene, hop-21(22)-ene, and hopan-22-ol. A putative gene in <i>Sch. japonicus</i> showed high similarity to bacterial squalene-hopene cyclase (SHC) genes and in particular to those of <i>Acetobacter</i> species. No orthologs of the putative <i>Sch. japonicus</i> SHC were found in other yeast species. Expression of the <i>Sch. japonicus</i> SHC gene (<i>Sjshc1</i>) in <i>Saccharomyces cerevisiae</i> enabled hopanoid synthesis and stimulated anaerobic growth in sterol-free media, thus indicating that one or more of the hopanoids produced by SjShc1 could at least partially replace sterols. Use of hopanoids as sterol surrogates represents a previously unknown adaptation of eukaryotic cells to anaerobic growth. The fast anaerobic growth of <i>Sch. japonicus</i> in sterol-free media is an interesting trait for developing robust fungal cell factories for application in anaerobic industrial processes.