Ricci J N, Michel A J, Newman D K
Division of Biology and Biological Engineering, California Institute of Technology, Pasadena, CA, USA.
Geobiology. 2015 May;13(3):267-77. doi: 10.1111/gbi.12129. Epub 2015 Jan 29.
Hopanoids are bacterial steroid-like lipids that can be preserved in the rock record on billion-year timescales. 2-Methylhopanoids are of particular interest to geobiologists because methylation is one of the few chemical modifications that remain after diagenesis and catagenesis. 2-Methylhopanes, the molecular fossils of 2-methylhopanoids, are episodically enriched in the rock record, but we do not have a robust interpretation for their abundance patterns. Here, we exploit the evolutionary record found in molecular sequences from extant organisms to reconstruct the biosynthetic history of 2-methylhopanoids using the C-2 hopanoid methylase, HpnP. Based on HpnP phylogenetic analysis, we find that 2-methylhopanoids originated in a subset of the Alphaproteobacteria. This conclusion is statistically robust and reproducible in multiple trials varying the outgroup, trimming stringency, and ingroup dataset used to infer the evolution of this protein family. The capacity for 2-methylhopanoid production was likely horizontally transferred from the Alphaproteobacteria into the Cyanobacteria after the Cyanobacteria's major divergences. Together, these results suggest that the ancestral function of 2-methylhopanoids was not related to oxygenic photosynthesis but instead to a trait already present in the Alphaproteobacteria. Moreover, given that early 2-methylhopane deposits could have been made solely by Alphaproteobacteria before the acquisition of hpnP by Cyanobacteria, and that the Alphaproteobacteria are thought to be ancestrally aerobic, we infer that 2-methylhopanoids likely arose after the oxygenation of the atmosphere. This finding is consistent with the geologic record-the oldest syngenetic 2-methylhopanes occur after the rise of oxygen, in middle Proterozoic strata of the Barney Creek Formation.
藿烷类是细菌类固醇样脂质，可在数十亿年的时间尺度上保存在岩石记录中。2-甲基藿烷类对地球生物学家特别有意义，因为甲基化是成岩作用和深成作用后仍保留的少数化学修饰之一。2-甲基藿烷作为2-甲基藿烷类的分子化石，在岩石记录中偶尔会富集，但我们对它们的丰度模式没有一个可靠的解释。在这里，我们利用现存生物分子序列中的进化记录，使用C-2藿烷类甲基转移酶HpnP来重建2-甲基藿烷类的生物合成历史。基于HpnP系统发育分析，我们发现2-甲基藿烷类起源于α-变形菌的一个子集。这一结论在统计学上是可靠的，并且在多个试验中都是可重复的，这些试验改变了用于推断该蛋白质家族进化的外类群、修剪严格度和内类群数据集。2-甲基藿烷类的生产能力可能在蓝细菌发生主要分化后从α-变形菌水平转移到蓝细菌中。总之，这些结果表明，2-甲基藿烷类的原始功能与氧光合作用无关，而是与α-变形菌中已经存在的一种特性有关。此外，鉴于在蓝细菌获得hpnP之前，早期的2-甲基藿烷沉积物可能仅由α-变形菌形成，并且α-变形菌被认为在祖先时期是需氧的，我们推断2-甲基藿烷类可能在大气氧化之后出现。这一发现与地质记录一致——最古老的同生2-甲基藿烷出现在氧气上升之后，在巴尼溪组的中元古代地层中。
