The Tricarboxylic Acid Cycle in Rhodospirillum Rubrum

The oxidation of acetic acid in the dark by the non-sulfur purple bacterium, Rhodospirillum rubrum, was shown by van Niel (1, 2) to require the presence of either carbon dioxide or trace amounts of the Cb-dicarboxylic acids of the Krebs cycle. These results could readily be interpreted to mean that the oxidation of acetic acid proceeds by way of the Krebs cycle, the presence of COZ being required to maintain a sufficient concentration of the Cd acids. However, cu-ketoglutaric, citric, and isocitric acids were unable to spark acetic acid oxidation in this organism. In addition, citric acid could not be utilized as the sole carbon source for growth. The latter findings would imply either that the Krebs cycle is inoperative in R. rubrum for the terminal oxidation of acetic acid or that the cell membrane is impermeable to a-ketoglutaric, citric, and isocitric acids. The inability of yeast and Escherichia coli to utilize citric acid was interpreted by Krebs and Johnson (3,4) as evidence against the functioning of a tricarboxylic acid cycle. Karlsson and Barker (5) were unable to demonstrate a tricarboxylic acid cycle in Axotobacter agilis for the terminal oxidation of acetic acid. More recently Ajl and coworkers (6-9) claimed to have demonstrated a G-dicarboxylic acid cycle in E. coli for the terminal oxidation of acetic acid which enters the cycle by a Wieland-Thunberg condensation. In Aerobacter aerogenes, on the other hand, they found that the tricarboxylic acid and the Ch-dicarboxylic acid cycles are coexistent, whereas in Micrococcus lysodeilcticus only the tricarboxylic acid cycle operates. While the evidence mentioned above would indicate that the Krebs cycle is not the major pathway for the oxidation of acetic acid by microorganisms, evidence to the contrary has also accumulated. Thus it was found by Lynen and Necuillah (lo), Campbell and Stokes (II), and Lara and Stokes (12) that the inability of yeast, Pseudomonas aeruginosa, and E. coli, respectively, to utilize citric acid may be due to the impermeability of t.he cell membrane to this substance. A comparable explanation of