Department of Earth System Science, Stanford University, Stanford, CA, 94305, USA.
Department of Chemistry, State University of New York-Environmental Science and Forestry, Syracuse, NY, 13210, USA.
Nat Commun. 2023 Apr 3;14(1):1859. doi: 10.1038/s41467-023-37552-3.
Sterane molecular fossils are broadly interpreted as eukaryotic biomarkers, although diverse bacteria also produce sterols. Steranes with side-chain methylations can act as more specific biomarkers if their sterol precursors are limited to particular eukaryotes and are absent in bacteria. One such sterane, 24-isopropylcholestane, has been attributed to demosponges and potentially represents the earliest evidence for animals on Earth, but enzymes that methylate sterols to give the 24-isopropyl side-chain remain undiscovered. Here, we show that sterol methyltransferases from both sponges and yet-uncultured bacteria function in vitro and identify three methyltransferases from symbiotic bacteria each capable of sequential methylations resulting in the 24-isopropyl sterol side-chain. We demonstrate that bacteria have the genomic capacity to synthesize side-chain alkylated sterols, and that bacterial symbionts may contribute to 24-isopropyl sterol biosynthesis in demosponges. Together, our results suggest bacteria should not be dismissed as potential contributing sources of side-chain alkylated sterane biomarkers in the rock record.
甾烷类分子化石被广泛解释为真核生物生物标志物，尽管多种细菌也会产生甾醇。如果甾烷类化合物的甾醇前体仅限于特定的真核生物，并且在细菌中不存在，则具有侧链甲基化的甾烷类化合物可以作为更具特异性的生物标志物。一种这样的甾烷，24-异丙基胆甾烷，被归因于海绵动物，并且可能代表地球上最早的动物证据，但是将甾醇甲基化为 24-异丙基侧链的酶仍然未被发现。在这里，我们表明来自海绵动物和尚未培养的细菌的甾醇甲基转移酶在体外起作用，并从共生细菌中鉴定出三种甲基转移酶，每种都能够进行连续甲基化，从而产生 24-异丙基甾醇侧链。我们证明了细菌具有合成侧链烷基化甾醇的基因组能力，并且细菌共生体可能有助于海绵动物中 24-异丙基甾醇的生物合成。总之，我们的结果表明，不应将细菌排除为岩石记录中侧链烷基化甾烷生物标志物的潜在来源。
