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Walters KA, Redding KE, Golbeck JH. Identification and characterization of the low molecular mass ferredoxins involved in central metabolism in Heliomicrobium modesticaldum. PHOTOSYNTHESIS RESEARCH 2024:10.1007/s11120-023-01069-z. [PMID: 38306001 DOI: 10.1007/s11120-023-01069-z] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/19/2023] [Accepted: 12/11/2023] [Indexed: 02/03/2024]
Abstract
The homodimeric Type I reaction center (RC) from Heliomicrobium modesticaldum lacks the PsaC subunit found in Photosystem I and instead uses the interpolypeptide [4Fe-4S] cluster FX as the terminal electron acceptor. Our goal was to identify which of the small mobile dicluster ferredoxins encoded by the H. modesticaldum genome are capable of accepting electrons from the heliobacterial RC (HbRC) and pyruvate:ferredoxin oxidoreductase (PFOR), a key metabolic enzyme. Analysis of the genome revealed seven candidates: HM1_1462 (PshB1), HM1_1461 (PshB2), HM1_2505 (Fdx3), HM1_0869 (FdxB), HM1_1043, HM1_0357, and HM1_2767. Heterologous expression in Escherichia coli and studies using time-resolved optical spectroscopy revealed that only PshB1, PshB2, and Fdx3 are capable of accepting electrons from the HbRC and PFOR. Modeling studies using AlphaFold show that only PshB1, PshB2, and Fdx3 should be capable of docking on PFOR at a positively charged patch that overlays a surface-proximal [4Fe-4S] cluster. Proteomic analysis of wild-type and gene deletion strains ΔpshB1, ΔpshB2, ΔpshB1pshB2, and Δfdx3 grown under nitrogen-replete conditions revealed that Fdx3 is undetectable in the wild-type, ΔpshB1, and Δfdx3 strains, but it is present in the ΔpshB2 and ΔpshB1pshB2 strains, implying that Fdx3 may substitute for PshB2. When grown under nitrogen-deplete conditions, Fdx3 is present in the wild-type and all deletion strains except for Δfdx3. None of the knockout strains demonstrated significant impairment during chemotrophic dark growth on pyruvate, photoheterotrophic light growth on pyruvate, or phototrophic growth on acetate+CO2, indicating a high degree of redundancy among these three electron transfer proteins. Loss of both PshB1 and PshB2, but not FdxB, resulted in poor growth under N2-fixing conditions.
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Affiliation(s)
- Karim A Walters
- Department of Biochemistry and Molecular Biology, The Pennsylvania State University, University Park, PA, 16802, USA
| | - Kevin E Redding
- School of Molecular Sciences, Arizona State University, 1711 S Rural Rd, Box 871604, Tempe, AZ, 85287-1604, USA.
- Center for Bioenergy & Photosynthesis, Arizona State University, Tempe, AZ, 85287, USA.
| | - John H Golbeck
- Department of Biochemistry and Molecular Biology, The Pennsylvania State University, University Park, PA, 16802, USA.
- Department of Chemistry, The Pennsylvania State University, University Park, PA, 16802, USA.
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Orf GS, Gisriel CJ, Granstrom J, Baker PL, Redding KE. The PshX subunit of the photochemical reaction center from Heliobacterium modesticaldum acts as a low-energy antenna. PHOTOSYNTHESIS RESEARCH 2022; 151:11-30. [PMID: 34480322 DOI: 10.1007/s11120-021-00871-x] [Citation(s) in RCA: 6] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/25/2021] [Accepted: 08/09/2021] [Indexed: 06/13/2023]
Abstract
The anoxygenic phototrophic bacterium Heliobacterium modesticaldum contains a photochemical reaction center protein complex (called the HbRC) consisting of a homodimer of the PshA polypeptide and two copies of a newly discovered polypeptide called PshX, which is a single transmembrane helix that binds two bacteriochlorophyll g molecules. To assess the function of PshX, we produced a ∆pshX strain of Hbt. modesticaldum by leveraging the endogenous Hbt. modesticaldum Type I-A CRISPR-Cas system to aid in mutant selection. We optimized this system by separating the homologous recombination and CRISPR-based selection steps into two plasmid transformations, allowing for markerless gene replacement. Fluorescence and low-temperature absorbance of the purified HbRC from the wild-type and ∆pshX strains showed that the bacteriochlorophylls bound by PshX have the lowest site energies in the entire HbRC. This indicates that PshX acts as a low-energy antenna subunit, participating in entropy-assisted uphill energy transfer toward the P800 special bacteriochlorophyll g pair. We further discuss the role that PshX may play in stability of the HbRC, its conservation in other heliobacterial species, and the evolutionary pressure to produce and maintain single-TMH subunits in similar locations in other reaction centers.
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Affiliation(s)
- Gregory S Orf
- Center for Bioenergy and Photosynthesis, School of Molecular Sciences, Arizona State University, Tempe, AZ, 85287, USA
- Infectious Disease Research, Abbott Diagnostics Division, Abbott Park, IL, 60064, USA
| | - Christopher J Gisriel
- Center for Bioenergy and Photosynthesis, School of Molecular Sciences, Arizona State University, Tempe, AZ, 85287, USA
- Department of Chemistry, Yale University, New Haven, CT, 06520, USA
| | - Jesse Granstrom
- Center for Bioenergy and Photosynthesis, School of Molecular Sciences, Arizona State University, Tempe, AZ, 85287, USA
| | - Patricia L Baker
- Center for Bioenergy and Photosynthesis, School of Molecular Sciences, Arizona State University, Tempe, AZ, 85287, USA
| | - Kevin E Redding
- Center for Bioenergy and Photosynthesis, School of Molecular Sciences, Arizona State University, Tempe, AZ, 85287, USA.
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3
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Dewey ED, Stokes LM, Burchell BM, Shaffer KN, Huntington AM, Baker JM, Nadendla S, Giglio MG, Bender KS, Touchman JW, Blankenship RE, Madigan MT, Sattley WM. Analysis of the Complete Genome of the Alkaliphilic and Phototrophic Firmicute Heliorestis convoluta Strain HH T. Microorganisms 2020; 8:E313. [PMID: 32106460 PMCID: PMC7143216 DOI: 10.3390/microorganisms8030313] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/26/2020] [Revised: 02/16/2020] [Accepted: 02/22/2020] [Indexed: 11/16/2022] Open
Abstract
Despite significant interest and past work to elucidate the phylogeny and photochemistry of species of the Heliobacteriaceae, genomic analyses of heliobacteria to date have been limited to just one published genome, that of the thermophilic species Heliobacterium (Hbt.) modesticaldum str. Ice1T. Here we present an analysis of the complete genome of a second heliobacterium, Heliorestis (Hrs.) convoluta str. HHT, an alkaliphilic, mesophilic, and morphologically distinct heliobacterium isolated from an Egyptian soda lake. The genome of Hrs. convoluta is a single circular chromosome of 3.22 Mb with a GC content of 43.1% and 3263 protein-encoding genes. In addition to culture-based observations and insights gleaned from the Hbt. modesticaldum genome, an analysis of enzyme-encoding genes from key metabolic pathways supports an obligately photoheterotrophic lifestyle for Hrs. convoluta. A complete set of genes encoding enzymes for propionate and butyrate catabolism and the absence of a gene encoding lactate dehydrogenase distinguishes the carbon metabolism of Hrs. convoluta from its close relatives. Comparative analyses of key proteins in Hrs. convoluta, including cytochrome c553 and the Fo alpha subunit of ATP synthase, with those of related species reveal variations in specific amino acid residues that likely contribute to the success of Hrs. convoluta in its highly alkaline environment.
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Affiliation(s)
- Emma D. Dewey
- Division of Natural Sciences, Indiana Wesleyan University, Marion, IN 46953, USA; (E.D.D.); (L.M.S.); (B.M.B.); (K.N.S.); (A.M.H.); (J.M.B.)
| | - Lynn M. Stokes
- Division of Natural Sciences, Indiana Wesleyan University, Marion, IN 46953, USA; (E.D.D.); (L.M.S.); (B.M.B.); (K.N.S.); (A.M.H.); (J.M.B.)
| | - Brad M. Burchell
- Division of Natural Sciences, Indiana Wesleyan University, Marion, IN 46953, USA; (E.D.D.); (L.M.S.); (B.M.B.); (K.N.S.); (A.M.H.); (J.M.B.)
| | - Kathryn N. Shaffer
- Division of Natural Sciences, Indiana Wesleyan University, Marion, IN 46953, USA; (E.D.D.); (L.M.S.); (B.M.B.); (K.N.S.); (A.M.H.); (J.M.B.)
| | - Austin M. Huntington
- Division of Natural Sciences, Indiana Wesleyan University, Marion, IN 46953, USA; (E.D.D.); (L.M.S.); (B.M.B.); (K.N.S.); (A.M.H.); (J.M.B.)
| | - Jennifer M. Baker
- Division of Natural Sciences, Indiana Wesleyan University, Marion, IN 46953, USA; (E.D.D.); (L.M.S.); (B.M.B.); (K.N.S.); (A.M.H.); (J.M.B.)
| | - Suvarna Nadendla
- Institute for Genome Sciences, University of Maryland School of Medicine, Baltimore, MD 21201, USA; (S.N.); (M.G.G.)
| | - Michelle G. Giglio
- Institute for Genome Sciences, University of Maryland School of Medicine, Baltimore, MD 21201, USA; (S.N.); (M.G.G.)
| | - Kelly S. Bender
- Department of Microbiology, Southern Illinois University, Carbondale, IL 62901, USA; (K.S.B.); (M.T.M.)
| | | | - Robert E. Blankenship
- Departments of Biology and Chemistry, Washington University in Saint Louis, St. Louis, MO 63130, USA;
| | - Michael T. Madigan
- Department of Microbiology, Southern Illinois University, Carbondale, IL 62901, USA; (K.S.B.); (M.T.M.)
| | - W. Matthew Sattley
- Division of Natural Sciences, Indiana Wesleyan University, Marion, IN 46953, USA; (E.D.D.); (L.M.S.); (B.M.B.); (K.N.S.); (A.M.H.); (J.M.B.)
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4
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Baker PL, Orf GS, Kevershan K, Pyne ME, Bicer T, Redding KE. Using the Endogenous CRISPR-Cas System of Heliobacterium modesticaldum To Delete the Photochemical Reaction Center Core Subunit Gene. Appl Environ Microbiol 2019; 85:e01644-19. [PMID: 31540988 PMCID: PMC6856316 DOI: 10.1128/aem.01644-19] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/19/2019] [Accepted: 09/14/2019] [Indexed: 11/20/2022] Open
Abstract
In Heliobacterium modesticaldum, as in many Firmicutes, deleting genes by homologous recombination using standard techniques has been extremely difficult. The cells tend to integrate the introduced plasmid into the chromosome by a single recombination event rather than perform the double recombination required to replace the targeted locus. Transformation with a vector containing only a homologous recombination template for replacement of the photochemical reaction center gene pshA produced colonies with multiple genotypes, rather than a clean gene replacement. To address this issue, we required an additional means of selection to force a clean gene replacement. In this study, we report the genetic structure of the type I-A and I-E CRISPR-Cas systems from H. modesticaldum, as well as methods to leverage the type I-A system for genome editing. In silico analysis of the CRISPR spacers revealed a potential consensus protospacer adjacent motif (PAM) required for Cas3 recognition, which was then tested using an in vivo interference assay. Introduction of a homologous recombination plasmid that carried a miniature CRISPR array targeting sequences in pshA (downstream of a naturally occurring PAM sequence) produced nonphototrophic transformants with clean replacements of the pshA gene with ∼80% efficiency. Mutants were confirmed by PCR, sequencing, optical spectroscopy, and growth characteristics. This methodology should be applicable to any genetic locus in the H. modesticaldum genome.IMPORTANCE The heliobacteria are the only phototrophic members of the largely Gram-positive phylum Firmicutes, which contains medically and industrially important members, such as Clostridium difficile and Clostridium acetobutylicum Heliobacteria are of interest in the study of photosynthesis because their photosynthetic system is unique and the simplest known. Since their discovery in the early 1980s, work on the heliobacteria has been hindered by the lack of a genetic transformation system. The problem of introducing foreign DNA into these bacteria has been recently rectified by our group; however, issues still remained for efficient genome editing. The significance of this work is that we have characterized the endogenous type I CRISPR-Cas system in the heliobacteria and leveraged it to assist in genome editing. Using the CRISPR-Cas system allowed us to isolate transformants with precise replacement of the pshA gene encoding the main subunit of the photochemical reaction center.
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Affiliation(s)
- Patricia L Baker
- School of Molecular Sciences, Arizona State University, Tempe, Arizona, USA
- Center for Bioenergy and Photosynthesis, Arizona State University, Tempe, Arizona, USA
| | - Gregory S Orf
- School of Molecular Sciences, Arizona State University, Tempe, Arizona, USA
- Center for Bioenergy and Photosynthesis, Arizona State University, Tempe, Arizona, USA
| | - Kimberly Kevershan
- School of Molecular Sciences, Arizona State University, Tempe, Arizona, USA
| | - Michael E Pyne
- Department of Biology, Centre for Applied Synthetic Biology, Concordia University, Montréal, Québec, Canada
| | - Taner Bicer
- School of Molecular Sciences, Arizona State University, Tempe, Arizona, USA
| | - Kevin E Redding
- School of Molecular Sciences, Arizona State University, Tempe, Arizona, USA
- Center for Bioenergy and Photosynthesis, Arizona State University, Tempe, Arizona, USA
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A Molecular Biology Tool Kit for the Phototrophic Firmicute Heliobacterium modesticaldum. Appl Environ Microbiol 2019; 85:AEM.01287-19. [PMID: 31375483 DOI: 10.1128/aem.01287-19] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/06/2019] [Accepted: 07/21/2019] [Indexed: 11/20/2022] Open
Abstract
The heliobacteria are members of the bacterial order Clostridiales and form the only group of phototrophs in the phylum Firmicutes Several physiological and metabolic characteristics make them an interesting subject of investigation, including their minimalist photosynthetic system, nitrogen fixation abilities, and ability to reduce toxic metals. While the species Heliobacterium modesticaldum is an excellent candidate as a model system for the family Heliobacteriaceae, since an annotated genome and transcriptomes are available, studies in this organism have been hampered by the lack of genetic tools. We adapted techniques for genetic manipulation of related clostridial species for use with H. modesticaldum Five heliobacterial DNA methyltransferase genes were expressed in an Escherichia coli strain engineered as a conjugative plasmid donor for broad-host-range plasmids. Premethylation of the shuttle vectors before conjugation into H. modesticaldum is absolutely required for production of transconjugant colonies. The introduced shuttle vectors are maintained stably and can be recovered using a modified minipreparation procedure developed to inhibit endogenous DNase activity. Furthermore, we describe the formulation of various growth media, including a defined medium for metabolic studies and isolation of auxotrophic mutants.IMPORTANCE Heliobacteria are anoxygenic phototrophic bacteria with the simplest known photosynthetic apparatus. They are unique in using bacteriochlorophyll g as their main pigment and lacking a peripheral antenna system. Until now, research on this organism has been hampered by the lack of a genetic transformation system. Without such a system, gene knockouts, site-directed mutations, and gene expression studies cannot be performed to help us further understand or manipulate the organism. Here we report the genetic transformation of a heliobacterium, which should enable future genetic studies in this unique phototrophic organism.
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Yang JH, Sarrou I, Martin-Garcia JM, Zhang S, Redding KE, Fromme P. Purification and biochemical characterization of the ATP synthase from Heliobacterium modesticaldum. Protein Expr Purif 2015; 114:1-8. [PMID: 25979464 DOI: 10.1016/j.pep.2015.05.006] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/21/2015] [Revised: 04/28/2015] [Accepted: 05/06/2015] [Indexed: 11/26/2022]
Abstract
Heliobacterium modesticaldum is an anaerobic photosynthetic bacterium that grows optimally at pH 6-7 and 52°C and is the only phototrophic member of the Firmicutes phylum family (gram-positive bacteria with low GC content). The ATP synthase of H. modesticaldum was isolated and characterized at the biochemical and biophysical levels. The isolated holoenzyme exhibited the subunit patterns of F-type ATP synthases containing a 5-subunit hydrophilic F1 subcomplex and a 3-subunit hydrophobic F0 subcomplex. ATP hydrolysis by the isolated HF1F0 ATP synthase was successfully detected after pretreatment with different detergents by an in-gel ATPase activity assay, which showed that the highest activity was detected in the presence of mild detergents such as LDAO; moreover, high catalytic activity in the gel was already detected after the initial incubation period of 0.5h. In contrast, HF1F0 showed extremely low ATPase activity in harsher detergents such as TODC. The isolated fully functional enzyme will form the basis for future structural studies.
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Affiliation(s)
- Jay-How Yang
- Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287-1604, USA; Center for Applied Structural Discovery, The Biodesign Institute, Arizona State University, Tempe, AZ 85287-1604, USA
| | - Iosifina Sarrou
- Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287-1604, USA; Institute of Molecular Biology & Biotechnology, Foundation for Research & Technology-Hellas, Nikolaou Plastira 100, GR-70013 Heraklion, Crete, Greece
| | - Jose M Martin-Garcia
- Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287-1604, USA; Center for Applied Structural Discovery, The Biodesign Institute, Arizona State University, Tempe, AZ 85287-1604, USA
| | - Shangji Zhang
- School of Life Sciences, Arizona State University, Tempe, AZ 85287-1604, USA
| | - Kevin E Redding
- Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287-1604, USA
| | - Petra Fromme
- Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287-1604, USA; Center for Applied Structural Discovery, The Biodesign Institute, Arizona State University, Tempe, AZ 85287-1604, USA.
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7
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Zhang Y, Majumder ELW, Yue H, Blankenship RE, Gross ML. Structural analysis of diheme cytochrome c by hydrogen-deuterium exchange mass spectrometry and homology modeling. Biochemistry 2014; 53:5619-30. [PMID: 25138816 PMCID: PMC4159202 DOI: 10.1021/bi500420y] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/17/2023]
Abstract
![]()
A lack
of X-ray or nuclear magnetic resonance structures of proteins
inhibits their further study and characterization, motivating the
development of new ways of analyzing structural information without
crystal structures. The combination of hydrogen–deuterium exchange
mass spectrometry (HDX-MS) data in conjunction with homology modeling
can provide improved structure and mechanistic predictions. Here a
unique diheme cytochrome c (DHCC) protein from Heliobacterium modesticaldum is studied with both HDX and homology modeling to bring some definition of the structure of the
protein and its role. Specifically, HDX data were used to guide the
homology modeling to yield a more functionally relevant structural
model of DHCC.
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Affiliation(s)
- Ying Zhang
- Department of Chemistry, Washington University in St. Louis , St. Louis, Missouri 63130, United States
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Energy Conservation in Heliobacteria: Photosynthesis and Central Carbon Metabolism. THE STRUCTURAL BASIS OF BIOLOGICAL ENERGY GENERATION 2014. [DOI: 10.1007/978-94-017-8742-0_13] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/25/2023]
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Gupta RS. Origin and Spread of Photosynthesis Based upon Conserved Sequence Features in Key Bacteriochlorophyll Biosynthesis Proteins. Mol Biol Evol 2012; 29:3397-412. [DOI: 10.1093/molbev/mss145] [Citation(s) in RCA: 27] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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Tang KH, Tang YJ, Blankenship RE. Carbon metabolic pathways in phototrophic bacteria and their broader evolutionary implications. Front Microbiol 2011; 2:165. [PMID: 21866228 PMCID: PMC3149686 DOI: 10.3389/fmicb.2011.00165] [Citation(s) in RCA: 71] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/06/2011] [Accepted: 07/18/2011] [Indexed: 11/19/2022] Open
Abstract
Photosynthesis is the biological process that converts solar energy to biomass, bio-products, and biofuel. It is the only major natural solar energy storage mechanism on Earth. To satisfy the increased demand for sustainable energy sources and identify the mechanism of photosynthetic carbon assimilation, which is one of the bottlenecks in photosynthesis, it is essential to understand the process of solar energy storage and associated carbon metabolism in photosynthetic organisms. Researchers have employed physiological studies, microbiological chemistry, enzyme assays, genome sequencing, transcriptomics, and (13)C-based metabolomics/fluxomics to investigate central carbon metabolism and enzymes that operate in phototrophs. In this report, we review diverse CO(2) assimilation pathways, acetate assimilation, carbohydrate catabolism, the tricarboxylic acid cycle and some key, and/or unconventional enzymes in central carbon metabolism of phototrophic microorganisms. We also discuss the reducing equivalent flow during photoautotrophic and photoheterotrophic growth, evolutionary links in the central carbon metabolic network, and correlations between photosynthetic and non-photosynthetic organisms. Considering the metabolic versatility in these fascinating and diverse photosynthetic bacteria, many essential questions in their central carbon metabolism still remain to be addressed.
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Affiliation(s)
- Kuo-Hsiang Tang
- Department of Biology, Washington University in St. LouisSt. Louis, MO, USA
- Department of Chemistry, Washington University in St. LouisSt. Louis, MO, USA
| | - Yinjie J. Tang
- Department of Energy, Environment, and Chemical Engineering, Washington University in St. LouisSt. Louis, MO, USA
| | - Robert Eugene Blankenship
- Department of Biology, Washington University in St. LouisSt. Louis, MO, USA
- Department of Chemistry, Washington University in St. LouisSt. Louis, MO, USA
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Tang KH, Yue H, Blankenship RE. Energy metabolism of Heliobacterium modesticaldum during phototrophic and chemotrophic growth. BMC Microbiol 2010; 10:150. [PMID: 20497547 PMCID: PMC2887804 DOI: 10.1186/1471-2180-10-150] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/04/2010] [Accepted: 05/24/2010] [Indexed: 11/10/2022] Open
Abstract
Background Heliobacterium modesticaldum is a gram-positive nitrogen-fixing phototrophic bacterium that can grow either photoheterotrophically or chemotrophically but not photoautotrophically. Surprisingly, this organism is lacking only one gene for the complete reverse tricarboxylic acid (rTCA) cycle required for autotrophic carbon fixation. Along with the genomic information reported recently, we use multiple experimental approaches in this report to address questions regarding energy metabolic pathways in darkness, CO2 fixation, sugar assimilation and acetate metabolism. Results We present the first experimental evidence that D-ribose, D-fructose and D-glucose can be photoassimilated by H. modesticaldum as sole carbon sources in newly developed defined growth medium. Also, we confirm two non-autotrophic CO2-fixation pathways utilized by H. modesticaldum: reactions catalyzed by pyruvate:ferredoxin oxidoreductase and phosphoenolpyruvate carboxykinase, and report acetate excretion during phototrophic and chemotrophic growth. Further, genes responsible for pyruvate fermentation, which provides reducing power for nitrogen assimilation, carbon metabolism and hydrogen production, are either active or up-regulated during chemotrophic growth. The discovery of ferredoxin-NADP+ oxidoreductase (FNR) activity in cell extracts provides the reducing power required for carbon and nitrogen metabolisms. Moreover, we show that photosynthetic pigments are produced by H. modesticaldum during the chemotrophic growth, and demonstrate that H. modesticaldum performs nitrogen fixation during both phototrophic and chemotrophic growth. Conclusion Collectively, this report represents the first comprehensive studies for energy metabolism in heliobacteria, which have the simplest known photosynthetic machinery among the entire photosynthetic organisms. Additionally, our studies provide new and essential insights, as well as broaden current knowledge, on the energy metabolism of the thermophilic phototrophic bacterium H. modesticaldum during phototrophic and chemotrophic growth.
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Affiliation(s)
- Kuo-Hsiang Tang
- Department of Biology, Campus Box 1137, Washington University in St. Louis, One Brookings Drive, St. Louis, Missouri 63130, USA
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