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Abstract：
Hopanoids and carotenoids are two of the major isoprenoid-derived lipid classes in prokaryotes that have been proposed to have similar membrane ordering properties as sterols. Methylobacterium extorquens contains hopanoids and carotenoids in their outer membrane, making them an ideal system to investigate the role of isoprenoid lipids in surface membrane function and cellular fitness. By genetically knocking out hpnE and crtB we disrupted the production of squalene and phytoene in M. extorquens PA1, which are the presumed precursors for hopanoids and carotenoids respectively. Deletion of hpnE revealed that carotenoid biosynthesis utilizes squalene as a precursor resulting in pigmentation with a C30 backbone, rather than the previously predicted canonical C40 phytoene-derived pathway. Phylogenetic analysis suggested that M. extorquens may have acquired the C30 pathway through lateral gene transfer from Planctomycetes. Surprisingly, disruption of carotenoid synthesis did not generate any major growth or membrane biophysical phenotypes, but slightly increased sensitivity to oxidative stress. We further demonstrated that hopanoids but not carotenoids are essential for growth at higher temperatures, membrane permeability and tolerance of low divalent cation concentrations. These observations show that hopanoids and carotenoids serve diverse roles in the outer membrane of M. extorquens PA1.
摘要：
类胡萝卜素和类胡萝卜素是原核生物中两种主要的类异戊二烯衍生的脂质类别，已被认为具有与固醇相似的膜排序特性。Methylobacteriumexterquenscontainshopanoidsand胡萝卜素intheirexternalfilm,使它们成为研究类异戊二烯脂质在表面膜功能和细胞适应性中的作用的理想系统。通过基因敲除hpnE和crtB，我们破坏了M.exterquensPA1中角鲨烯和八烯的产生，它们分别是类胡萝卜素和类胡萝卜素的假定前体。hpnE的缺失表明，类胡萝卜素的生物合成利用角鲨烯作为前体，导致具有C30骨架的色素沉着。而不是先前预测的经典C40植物源途径。系统发育分析表明，M.extorquens可能已通过从平细胞的横向基因转移获得了C30途径。令人惊讶的是,类胡萝卜素合成的破坏不会产生任何主要的生长或膜生物物理表型，但对氧化应激的敏感性略有增加。我们进一步证明，在较高的温度下，类胡萝卜素而不是类胡萝卜素对生长至关重要，膜通透性和对低二价阳离子浓度的耐受性。这些观察结果表明，类胡萝卜素和类胡萝卜素在M.exterquensPA1的外膜中起着不同的作用。
