Carbon Flow of Heliobacteria Is Related More to Clostridia than to the Green Sulfur Bacteria |
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Authors: | Kuo-Hsiang Tang Xueyang Feng Wei-Qin Zhuang Lisa Alvarez-Cohen Robert E Blankenship Yinjie J Tang |
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Institution: | From the Departments of ‡Biology.;§Chemistry, and ;¶Energy, Environment, and Chemical Engineering, Washington University, St. Louis, Missouri 63130 and ;the ‖Department of Civil and Environmental Engineering, University of California, Berkeley, California 94720 |
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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 CO2-enhanced growth has not been understood. Here, we report studies to fill the knowledge gap of heliobacterial carbon metabolism. We confirm that the CO2-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. |
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Keywords: | Bacteria Isotopic Tracers Metabolism Metabolomics Photosynthesis Tricarboxylic Acid (TCA) Cycle Amino Acid Biosynthesis Citrate Synthase Fluoroacetate |
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