Origin and transport of bacterial hopanoids

Hopanoids are a class of steroid-like lipids that fortify bacterial membranes. Derivatives of hopanoids known as "geo-hopanes" are abundant in sediments and are signatures of ancient bacteria, yet there are conflicting views on their use as markers of specific taxa or environments. We analyzed the conservation of hopanoid biosynthesis across bacterial genomes using modern taxonomic tools. We find that hopanoids most likely originated in an ancestor of marine alphaproteobacteria, and that tolerance of high osmolarity is the most common feature of hopanoid-producing strains. Additionally, microsynteny with hopanoid-related loci revealed new hopanoid-associated gene families, which we term hpnT and hpnS. Structural predictions and the restriction of these gene families to diderms suggest that they participate in hopanoid trafficking, potentially forming a pathway analogous to that of Mla proteins in E. coli.

Hopanoids are lipids that mediate stress resistance in diverse modern bacteria. Fossilized hopanoids are abundant in sediments deposited up to 1.6 Gya and serve as molecular markers for ancient microbes. Understanding what these markers can tell us about the biology of ancient bacteria is challenging, and it relies on knowing how the ability to synthesize hopanoids evolved. In this study, we suggest that the earliest hopanoids are markers of marine alphaproteobacteria and uncover new potential hopanoid transport- and biosynthesis-associated genes.