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Hopanoids Play a Role in Membrane Integrity and Ph Homeostasis in Rhodopseudomonas Palustris TIE-1ᰔ† Paula V

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Hopanoids Play a Role in Membrane Integrity and Ph Homeostasis in Rhodopseudomonas Palustris TIE-1ᰔ† Paula V View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Caltech Authors JOURNAL OF BACTERIOLOGY, Oct. 2009, p. 6145–6156 Vol. 191, No. 19 0021-9193/09/$08.00ϩ0 doi:10.1128/JB.00460-09 Copyright © 2009, American Society for Microbiology. All Rights Reserved. Hopanoids Play a Role in Membrane Integrity and pH Homeostasis in Rhodopseudomonas palustris TIE-1ᰔ† Paula V. Welander,1 Ryan C. Hunter,1 Lichun Zhang,2 Alex L. Sessions,2 Roger E. Summons,3 and Dianne K. Newman1,3,4* Department of Biology, Massachusetts Institute of Technology, 77 Massachusetts Avenue, 68-380, Cambridge, Massachusetts 021391; Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, MC100-23, 1200 E. California Boulevard, Pasadena, California 911252; Department of Earth, Atmospheric and Planetary Science, Massachusetts Institute of Technology, 77 Massachusetts Avenue, E25-633, Cambridge, Massachusetts 021393; and Howard Hughes Medical Institute, 4 77 Massachusetts Avenue, 68-171, Cambridge, Massachusetts 02139 Downloaded from Received 3 April 2009/Accepted 6 July 2009 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 jb.asm.org 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 struc- ture, 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 at CALIFORNIA INSTITUTE OF TECHNOLOGY on September 25, 2009 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. Bacteriohopanepolyols (BHPs) are a class of pentacyclic structural variation in hopanoids, where some are methylated triterpenoids, generally referred to as hopanoids, that are at the C-2 or C-3 position and some are unsaturated, and the found in a variety of bacteria. Bacterial hopanoids have been functionalization of the side chain can differ greatly (59). To hypothesized to be surrogates of eukaryotic sterols primarily date, no genetic, biochemical, physiological, or environmental because of similarities in their biosynthesis and structure (Fig. studies have been done that explain the biological significance 1 A and B) (36). Sterols have been shown to play an important of these modifications. A few studies have proposed that ho- role in stabilizing eukaryotic cell membranes and regulating panoids enhance membrane stability and decrease membrane membrane fluidity, as well as providing resistance to ethanol permeability in some bacteria, including Alicyclobacillus acido- and heat shock stress (20, 57, 61). Sterols are produced by all caldarius (43), Zymomonas mobilis (19), Frankia sp. (4), and eukaryotes, and the membranes of animals, plants, and fungi Streptomyces coelicolor (44). However, these studies did not have been shown to contain different types of sterols possibly utilize a genetic approach or in vivo experiments to conclu- with different physiological roles (20). Usually, these sterols sively elucidate the role of hopanoids in membrane structure vary in their patterns of A-ring and side chain alkylation and and stability. Thus, the extent to which they are sterol surro- unsaturation (16, 20, 40). There are also a variety of sterol gates remains an open question. esters and steryl glycosides found in plants and yeast (41, 47). While understanding the function of hopanoids is interesting Regardless of which sterols are produced by eukaryotic cells, from a purely cell biological perspective, there is an additional all eukaryotic organisms require these molecules to survive motivation provided by evolutionary biology. Geochemists and (40). geobiologists frequently utilize these compounds as “molecular Hopanoids have been shown to localize to the cytoplasmic fossils” or biomarkers (8). In this context, biomarkers are or- and outer membranes of certain bacteria (21, 24, 54), and in ganic compounds that are preserved in ancient sedimentary vitro experiments have shown that these molecules are efficient rocks and can be linked to specific organisms by analogy with in condensing artificial membranes and enhancing the viscosity their production in modern organisms (8, 9). The use of these of liposomes (3, 43). As with eukaryotic sterols, there is also molecular fossils has proven to be a powerful approach to study and reconstruct both ancient and modern microbial eco- systems (8). Sedimentary hopanes comprise one of the most * Corresponding author. Mailing address: Department of Biology, abundant and ubiquitous types of extractable organic com- 77 Massachusetts Ave., 68-380, Cambridge, MA 02139. Phone: (617) pounds in ancient rocks (8) and are unambiguously recognized 324-2770. Fax: (617) 324-3972. E-mail: [email protected]. † Supplemental material for this article may be found at http://jb as the chemical fossils of BHPs (35). Because BHPs with an .asm.org/. extended side chain and 35 carbon atoms are known to be ᰔ Published ahead of print on 10 July 2009. produced only by bacteria, extended C31 to C35 sedimentary 6145 6146 WELANDER ET AL. J. BACTERIOL. dance by cyanobacteria, and taking this together with their prevalence in ancient marine petroleum source rocks, they were proposed as biomarkers not only for cyanobacteria, but also for oxygenic photosynthesis (49, 56). However, there is no evidence that functionally links 2-methylbacteriohopanepoly- ols (2-MeBHPs) with oxygenic photosynthesis in modern cya- nobacteria. Furthermore, the lack of studies addressing the regulation of the production of hopanoids in modern bacteria limits our ability to constrain these molecules to a specific bacterial group. An example is the recent discovery of signifi- cant 2-MeBHP production by the anoxygenic phototroph Rho- dopseudomonas palustris, which had been previously tested and was thought not to produce methylated hopanoids (31, 48). Downloaded from This demonstrated not only that organisms other than cya- nobacteria are able to make 2-MeBHPs, but also that the production of these molecules depends on the growth condi- tions (48). Given these ambiguities, there are inadequate data to fully interpret the sedimentary hopane record. To begin to under- jb.asm.org stand the physiological functions of hopanoids in photosyn- thetic bacteria and to compare these functions to those of sterols in eukaryotes, we generated and characterized a ho- at CALIFORNIA INSTITUTE OF TECHNOLOGY on September 25, 2009 panoid deletion mutant of the genetically tractable anoxygenic phototroph R. palustris TIE-1. MATERIALS AND METHODS Bacterial strains, media, and growth conditions. The bacterial strains used in this study are listed in Table 1. Escherichia coli strains were grown in lysogeny broth (LB) at 37°C. E. coli strain BW29427 was supplemented with 60 ␮M diaminopimelic acid. For aerobic chemoheterotrophic growth, R. palustris strains were grown in unbuffered YP medium (0.3% yeast extract, 0.3% peptone) at 30°C in the dark with shaking at 250 rpm. Buffered YP medium was prepared by adding 100 mM MES (4-morpholineethanosulfonic acid) for pH 4.5, 5, and 6; 100 mM MOPS (4-morpholionepropanesulfonic acid) for pH 7; or 100 mM bicine [N,N-bis(2-hydroyxethylglycine)] for pH 8 and pH 9 and adjusting the FIG. 1. Biosynthesis of hopanoids in R. palustris TIE-1. (A) Ho- medium to the desired pH with concentrated HCl or NaOH prior to autoclaving panoids produced by R. palustris TIE-1. For 32,33,34,35-bacterioho- it. For anaerobic phototrophic growth, R. palustris strains were grown in anaer- panetetrol, R is OH; for 35-amino-32,33,34-bacteriohopanetriol, R is obic bicarbonate-buffered freshwater (FW) medium (22). Unless otherwise 2 2 noted, FW medium was supplemented from sterile anoxic stock solutions with 2 NH2. (B) Structures of the sterol biosynthesis intermediate lanosterol and of cholesterol, demonstrating the structural similarity to ho- mM sodium acetate for photoheterotrophic growth and 2 mM sodium thiosulfate for photoautotrophic growth. All anaerobic cultures were flushed and pressur- panoids. (C) Conversion of squalene to the basic hopene structure by 2 2 the squalene-hopene cyclase. The Entrez Gene accession number for ized to 5 lb/in with N2/CO2 (80%/20%) and incubated at 30°C at 50 W/m the squalene-hopene cyclase gene is NC_011004.1. without shaking. For anaerobic photoheterotrophic growth at acidic and alkaline pH, FW medium was prepared without bicarbonate in the medium or CO2 in the headspace to prevent fluctuations in the pH. Anoxic acidic medium was buffered with 50 mM MES, and
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