Hopanoids Play a Role in Membrane Integrity and pH Homeostasis in Rhodopseudomonas palustris TIE-1

Abstract

Sedimentary hopanes are pentacyclic triterpenoids that serve as biomarker proxies for bacteria and certain bacterial metabolisms, such as oxygenic photosynthesis and aerobic methanotrophy. Their parent molecules, the bacteriohopanepolyols (BHPs), have been hypothesized to be the bacterial equivalent of sterols. However, the actual function of BHPs in bacterial cells is poorly understood. Here, we report the physiological study of a mutant in Rhodopseudomonas palustris TIE-1 that is unable to produce any hopanoids. The deletion of the gene encoding the squalene-hopene cyclase protein (Shc), which cyclizes squalene to the basic hopene structure, resulted in a strain that no longer produced any polycyclic triterpenoids. This strain was able to grow chemoheterotrophically, photoheterotrophically, and photoautotrophically, demonstrating that hopanoids are not required for growth under normal conditions. A severe growth defect, as well as significant morphological damage, was observed when cells were grown under acidic and alkaline conditions. Although minimal changes in shc transcript expression were observed under certain conditions of pH shock, the total amount of hopanoid production was unaffected; however, the abundance of methylated hopanoids significantly increased. This suggests that hopanoids may play an indirect role in pH homeostasis, with certain hopanoid derivatives being of particular importance.

Identification of a methylase required for 2-methylhopanoid production and implications for the interpretation of sedimentary hopanes

“…For hopanoid analysis, strains were grown to late stationary phase (7 d). Total lipid extracts were prepared by solvent extraction, derivatized as acetates, and analyzed by high temperature GC-MS utilizing a DB-XLB column as previously described (15). Compounds were identified by comparison of retention times and mass spectra to authentic compounds (tetrahymanol from Trimyema sp.…”

“…Because the majority of bacterial diversity has not been cultured (22) and because the production of 2-methylhopanopids varies under different growth conditions (15,20,23), it is likely that our assessment of the taxonomic distribution of these molecules is incomplete. As a result, there is inadequate data to unambiguously interpret sedimentary 2-methylhopanes as biomarkers for cyanobacteria or other taxa.…”

“…These hydrocarbon molecules are recognized as the diagenetic products of bacteriohopanepolyols, functionalized pentacyclic triterpenoids that are more generally referred to as hopanoids. Hopanoids are produced by a variety of bacteria and are thought to play a role in membrane integrity and stress tolerance (12)(13)(14)(15). Because the hopanoid hydrocarbon skeleton is resistant to biodegradation, and is readily incorporated and preserved in sediments, these molecules have long been considered "molecular fossils" for ancient bacteria (16).…”

Bacteriohopanepolyols in tropical soils and sediments from the Congo River catchment area

“…The difference could be due to local environmental parameters. For example, pH is known to affect BHP distributions (concentration normalised to TOC and relative abundance) in environmental samples (Kim et al, 2011) and in laboratory culture experiments where changes in the amount and/or type of BHPs produced are reported (Poralla et al, 1984; Welander et al, 2009;Schmerk et al, 2011). The pH did not correlate with soil marker BHP concentration (lg/g TOC; r s À0.600, p 0.002), R soil (r s À0.203, p 0.341) or R 0 soil (r s À0.266, p 0.209).…”

Identification and quantification of polyfunctionalized hopanoids by high temperature gas chromatography–mass spectrometry

“…In modern sediments, functionalized BHPs are being explored as diagnostic for varying clades of bacteria (reviewed by ).In rocks and oils,the relative abundance of different hopane isomers can provide information about both environmental conditions during deposition, as well as maturity (Seifert and Moldowan, 1980;Brassell et al, 1983;Peters and Moldowan, 1991;Peters et al, 2005). For ancient rocks, hopanoids methylated at C-2 or C-3 have received particular attention because they have been proposed as diagnostic markers for cyanobacteria and aerobic methylotrophs, respectively (Zundel and Rohmer, 1985a,b;Summons and Jahnke, 1990;Summons et al, 1994Summons et al, , 1999Farrimond et al, 2004).A current research focus lies in understanding the cellular function of BHPs in bacteria, the genetic basis for their biosynthesis and modification, and their distribution and evolutionary relationships among extant organisms (Fischer et al, 2005;Blumenberg et al, 2006;Fischer and Pearson, 2007;Rashby et al, 2007;Doughty et al, 2009;Welander et al, 2009Welander et al, , 2010Welander et al, , 2012Bradley et al, 2010).…”