title: 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.
authors.affiliation: Department of Biology, Washington University, St. Louis, Missouri 63130, USA.
authors.forename: Kuo-Hsiang
authors.initials: KH
authors.lastname: Tang
authors.forename: Xueyang
authors.initials: X
authors.lastname: Feng
authors.forename: Wei-Qin
authors.initials: WQ
authors.lastname: Zhuang
authors.forename: Lisa
authors.initials: L
authors.lastname: Alvarez-Cohen
authors.forename: Robert E
authors.initials: RE
authors.lastname: Blankenship
authors.forename: Yinjie J
authors.initials: YJ
authors.lastname: Tang
mesh_terms: D000154:Aconitate Hydratase / Q000378:metabolism; D000693:Anaerobiosis / Q000502:physiology; D001426:Bacterial Proteins / Q000378:metabolism; D002244:Carbon / Q000378:metabolism*; D002245:Carbon Dioxide / Q000378:metabolism; D019414:Chlorobi / Q000502:physiology*; D002950:Citrate (si)-Synthase / Q000378:metabolism; D019343:Citric Acid / Q000378:metabolism; D002952:Citric Acid Cycle / Q000502:physiology*; D003013:Clostridium / Q000502:physiology*; D007532:Isoleucine / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins; D007532:Isoleucine; D002245:Carbon Dioxide; D019343:Citric Acid; D002244:Carbon; D002950:Citrate (si)-Synthase; D000154:Aconitate Hydratase
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