Acetate and bicarbonate metabolism in photosynthetic bacteria.

Abstract

Three major routes have been described which account for the oxidation of acetate or the assimilation of acetate into cellular material in the purple photosynthetic bacteria. In illuminated suspensions of Rhodospirillum rubrurn under anaerobic conditions and in the absence of CO,, acetate is assimilated to ,8-hydroxybutyrate polymer (23). In anaerobic suspensions of R. rubrurn which are capable of evolving molecular He, acetate and C4 dicarboxylic acids are oxidized to COO with a concomitant evolution of H, by a light dependent anaerobic tricarboxylic acid cycle (12). In anaerobic illuminated suspensions of Chromatiurn, which have been grown on acetate as the sole carbon source, acetate is assimilated to protein and polysaccharide through the glyoxylate cycle (10). Several proposals have appeared which describe alternate routes for the photometabolism of acetate and bicarbonate and for the synthesis of amino acids from these compounds in the purple photosynthetic bacteria (1, 4, 8, 9, 13, 15, 16, 19). These proposals have been based upon the labeling pattern of the organic acids resulting from the utilization of acetate-Cl4 and bicarbonate-Cl4 in resting cells; the degradation pattern of radioactive glutamate and alanine which had been synthesized from acetate-C14 and bicarbonate-C14; and the finding of an enzyme in Chrontatiumn and R. rubrumn which forms a C5 branched organic acid from acetyl-coenzyme A and pyruvate. The reactions involved in these alternate pathways have not been elucidated because: 1) it has been difficult to find any definite pattern in the distribution of radioactivity in the organic acids as a consequence of the utilization of acetate-C14 or bicarbonate-C14; 2) no enzymatic reaction other than the reductive synthesis of pyruvate from acetyl-coenzyme A and the condensation of acetyl-coenzyme A and pyruvate have been found in extracts which would support an alternate pathway. In view of the suggested alternate routes for the utilization of acetate and bicarbonate in R. rutbrunt and the fact that in resting cells of this organism hydrogen production is absolutely dependent on carbon flow through a light depenident anaerobic tricarboxylic acid cycle and electron passage to bacteriochlorophyll (12), it is important to assess the contribution of the anaerobic tricarboxylic acid cycle in the light dependent synthesis of amino acids from acetate units and bicarbonate. Since fluoroacetate has been uised as an extremely valuable tool in elucidating the presence of a light dependent anaerobic tricarboxylic acid cycle in R. rubrum we have tested the effects of this inhibitor on the light depen(lent synthesis of amino acids. In anaerobic resting cells capable of synthesizing glutamic acid from acetate and bicarbonate in the light, the addition of fluoroacetate completely inhibits this synthesis with a concomitant accumulation of citrate. Examination of glutamic acid synthesis in soluble extracts shows that glutamate can be synthesized from acetate and bicarbonate through citric acid. A preliminary communication of these results has been reported (5).