title: Evidence for autotrophy via the reverse tricarboxylic acid cycle in the marine magnetotactic coccus strain MC-1.
abstract: Strain MC-1 is a marine, microaerophilic, magnetite-producing, magnetotactic coccus phylogenetically affiliated with the alpha-Proteobacteria. Strain MC-1 grew chemolithotrophically with sulfide and thiosulfate as electron donors with HCO3-/CO2 as the sole carbon source. Experiments with cells grown microaerobically in liquid with thiosulfate and H14CO3-/14CO2 showed that all cell carbon was derived from H14CO3-/14CO2 and therefore that MC-1 is capable of chemolithoautotrophy. Cell extracts did not exhibit ribulose-1,5-bisphosphate carboxylase-oxygenase (RubisCO) activity, nor were RubisCO genes found in the draft genome of MC-1. Thus, unlike other chemolithoautotrophic, magnetotactic bacteria, strain MC-1 does not appear to utilize the Calvin-Benson-Bassham cycle for autotrophy. Cell extracts did not exhibit carbon monoxide dehydrogenase activity, indicating that the acetyl-coenzyme A pathway also does not function in strain MC-1. The 13C content of whole cells of MC-1 relative to the 13C content of the inorganic carbon source (Deltadelta13C) was -11.4 per thousand. Cellular fatty acids showed enrichment of 13C relative to whole cells. Strain MC-1 cell extracts showed activities for several key enzymes of the reverse (reductive) tricarboxylic acid (rTCA) cycle including fumarate reductase, pyruvate:acceptor oxidoreductase and 2-oxoglutarate:acceptor oxidoreductase. Although ATP citrate lyase (another key enzyme of the rTCA cycle) activity was not detected in strain MC-1 using commonly used assays, cell extracts did cleave citrate, and the reaction was dependent upon the presence of ATP and coenzyme A. Thus, we infer the presence of an ATP-dependent citrate-cleaving mechanism. These results are consistent with the operation of the rTCA cycle in MC-1. Strain MC-1 appears to be the first known representative of the alpha-Proteobacteria to use the rTCA cycle for autotrophy.
authors.affiliation: Department of Biochemistry, Biophysics, and Molecular Biology, Iowa State University, Ames, Iowa 50011, USA.
authors.forename: Timothy J
authors.initials: TJ
authors.lastname: Williams
authors.forename: Chuanlun L
authors.initials: CL
authors.lastname: Zhang
authors.forename: James H
authors.initials: JH
authors.lastname: Scott
authors.forename: Dennis A
authors.initials: DA
authors.lastname: Bazylinski
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D020561:Alphaproteobacteria / Q000235:genetics / Q000254:growth & development / Q000378:metabolism*; D001639:Bicarbonates / Q000378:metabolism; D002245:Carbon Dioxide / Q000378:metabolism; D019343:Citric Acid / Q000378:metabolism; D002952:Citric Acid Cycle; D005227:Fatty Acids / Q000378:metabolism; D052203:Ferrosoferric Oxide / Q000378:metabolism; D005798:Genes, Bacterial; D008280:Magnetics; D008386:Marine Biology; D008969:Molecular Sequence Data; D005810:Multigene Family; D012273:Ribulose-Bisphosphate Carboxylase / Q000235:genetics / Q000378:metabolism; D013885:Thiosulfates / Q000378:metabolism
chemical_list: D001639:Bicarbonates; D005227:Fatty Acids; D013885:Thiosulfates; D002245:Carbon Dioxide; D019343:Citric Acid; D000255:Adenosine Triphosphate; D012273:Ribulose-Bisphosphate Carboxylase; D052203:Ferrosoferric Oxide