GENOMIC APPROACHES TO EXPLORING THE GOE

In this study we used a molecular clock approach and sterane data from sedimentary rocks, to improve constraints on the evolution of sterol biosynthesis. Although some simple sterols can be synthesized by a few bacteria, their complex counterparts with a modified ring system and a variety of side chains, are unique to eukaryotes. In addition, sterol biosynthesis is an oxygen-intensive process with either 10 or 11 moles of oxygen needed to make one mole of sterol. Accordingly, the presence of complex steranes in ancient sedimentary rocks not only signals the presence of eukaryotes, but also the existence of aerobic metabolic processes. Our results for squalene monooxygenase and oxidosqualene cyclase, the first two enzymes of the canonical sterol biosynthesis pathway, infer that stem eukaryotes shared functionally modern sterol biosynthesis genes with bacteria via horizontal gene transfer around 2.31 Gyr ago (Gold et al., 2017), concurrent with the most recent geochemical evidence for the Great Oxidation Event (Luo et al., 2016). Our results further indicate that simple sterol biosynthesis existed well before the diversification of living eukaryotes and substantially predating the oldest detected sterane biomarkers at approximately 1.64 Gyr ago (Brocks et al., 2005). Phylogenomics and molecular clock studies for proteins that catalyze subsequent biosynthetic reactions suggest that the complete sterol biosynthesis pathway may not have been in place until the Neoproterozoic.

References

Brocks, J. J. et al. Biomarker evidence for green and purple sulphur bacteria in astratified Palaeoproterozoic sea. Nature 437, 866 (2005).

Gold, D.A., et al., 2016. Sterol and Genomic Analyses Validate the Sponge Biomarker Hypothesis. Proceedings of the National Academy of Sciences. doi/10.1073/pnas.1512614113.

Gold, D.A. et al.2017. Paleoproterozoic sterol biosynthesis and the rise of oxygen. Nature 543, 420-423.

Luo, G. et al. Rapid oxidation of Earth’s atmosphere 2.33 billion years ago. Science Advances 2, e1600134, doi:10.1126/sciadv.1600134 (2016).