Carbon Flow of Heliobacteria Is Related More to Clostridia than to the Green Sulfur Bacteria

Abstract

The recently discovered heliobacteria are the only Gram-positive photosynthetic bacteria that have been cultured. One of the unique features of heliobacteria is that they have properties of both the photosynthetic green sulfur bacteria (containing the type I reaction center) and Clostridia (forming heat-resistant endospores). Most of the previous studies of heliobacteria, which are strict anaerobes and have the simplest known photosynthetic apparatus, have focused on energy and electron transfer processes. It has been assumed that like green sulfur bacteria, the major carbon flow in heliobacteria is through the (incomplete) reductive (reverse) tricarboxylic acid cycle, whereas the lack of CO 2 -enhanced growth has not been understood. Here, we report studies to fill the knowledge gap of heliobacterial carbon metabolism. We confirm that the CO 2 -anaplerotic pathway is active during phototrophic growth and that isoleucine is mainly synthesized from the citramalate pathway. Furthermore, to our surprise, our results suggest that the oxidative (forward) TCA cycle is operative and more active than the previously reported reductive (reverse) tricarboxylic acid cycle. Both isotopomer analysis and activity assays suggest that citrate is produced by a putative (Re)-citrate synthase and then enters the oxidative (forward) TCA cycle. Moreover, in contrast to (Si)-citrate synthase, (Re)-citrate synthase produces a different isomer of 2-fluorocitrate that is not expected to inhibit the activity of aconitase.Heliobacteria are a relatively newly discovered group of anaerobic photosynthetic bacteria. All of the cultured heliobacteria require organic carbon for anoxygenic growth, and several of the species can fix nitrogen (1, 2). Heliobacteria are the only cultured Gram-positive photosynthetic bacteria and are phylogenetically related to the bacterial phylum Firmicutes, such as the aerobic Bacillus and anaerobic Clostridia (1). Among 10 cultured heliobacteria (2), Heliobacterium modesticaldum is the only one that can grow at temperatures of Ͼ50°C. Madigan and co-workers (1, 3) reported that pyruvate, lactate, acetate (ϩHCO 3 Ϫ ), or yeast extract can support the phototrophic growth of H. modesticaldum, and our recent studies demonstrated that D-sugars can also support the growth of H. modesticaldum (4).An unusual feature of heliobacteria is that they have properties of both green sulfur bacteria (containing the type I reaction center) and Clostridia (forming heat-resistant endospores) (1). Heliobacteria have the simplest photosynthetic apparatus among the photosynthetic organisms (5), and most of the research on them has been focused on understanding the photosynthetic machinery and energy transfer processes (1, 6 -8).In contrast, our knowledge of phototrophic carbon metabolism by heliobacteria is still poorly developed.Only two reported studies have experimentally probed the carbon metabolism of heliobacteria; one is our recent study with H. modesticaldum (4), and the other is the 1994 report by Kelly and co-workers (9) on Hel...

Unique Central Carbon Metabolic Pathways and Novel Enzymes in Phototrophic Bacteria Revealed by Integrative Genomics, 13C-based Metabolomics and Fluxomics

“…Here we report some unique and/or significant central carbon metabolic pathways and novel enzymes; details can be found in our recent published studies Feng et al, 2010b;Tang and Blankenship, 2010;Tang et al, 2009a;Tang et al, 2010a;Tang et al, 2010b;Wu et al, 2010).…”

“…The methods for bacterial growth, physiological studies, gene expression profiles via quantitative realtime PCR (QRT-PCR) (Tang et al, 2009b) and enzyme assays of Roseobacter denitrificans (Tang et al, 2009a), Chlorobaculum tepidum (Tang and Blankenship, 2010;Tang et al, 2010a), Heliobacterium modesticaldum and Cyanothece sp. ATCC 51142 Wu et al, 2010) are described elsewhere.…”

Recent advances in mapping environmental microbial metabolisms through13C isotopic fingerprints

“…Furthermore, heliobacteria (a phylum of anaerobic anoxygenic phototrophic bacteria) were considered to have an incomplete reverse TCA cycle owing to their lack of a conventional CIT synthase. Recently, Tang et al [9] reported that the oxidative TCA route (OAA ! CIT !…”

“…The channelling can change isotopomer fingerprints because it may prevent the randomization of orientation for symmetric metabolites [83]. For example, the orientations of the TCA cycle metabolites (SUC and MAL) can be conserved in S. cerevisiae in the pathway from [4][5][6][7][8][9][10][11][12][13] C]a-ketoglutarate to OAA [84]. Because of the channelling of SUC/ fumarate formation enzymes (SucCoA synthetase, SUC dehydrogenase and fumarase), the 13 C fractions in the second and third carbon in OAA resulting from [4- C) have been used to investigate metabolite channelling and compartmentation in cytosolic and mitochondrial metabolite pools [82,85].…”

“…Thermoanaerobacter sp. strain X514 (a thermophilic ethanol producer) has been studied via 13 C experiments with [1][2][3][4][5][6][7][8][9][10][11][12][13] C]glucose. On the basis of an isotopic analysis of PYR (reflected by alanine) labelling, the oxidative PP pathway for glucose metabolism shows low activity in strain X514.…”