K-054. Metabolic Pathways in the Pleomorphic Metal-Reducing Organism Desulfovibrio africanus Strain PCS

R. Chakraborty1, Y. Tang1, F. Pingitore1, J. D. Keasling1,2, T. C. Hazen1;
1Lawrence Berkeley Natl. Lab., Berkeley, CA, 2Univ. of California, Berkeley, CA.

Desulfovibrio africanus is a strict anaerobic sulfate-reducing organism that can also reduce toxic metals like Chromium (VI) to non-toxic Chromium (IV). While a lot of researchers have focused on studying the biochemistry, isolation and structure of metalloproteins in this organism, not much is known about the physiology of this organism. Previously, Desulfovibrio africanus strain PCS (99% similar to the type strain) was isolated from sediments in San Diego as a lactate oxidizing sulfate reducer. Using lactate as the sole electron donor, this organism could reduce almost 200μM Chromium(VI) even when subjected to stress by 50mM nitrate. Strain PCS appears as thin spiral shaped bacterium in mid log phase but morphs into spherical form later in the growth phase. It was observed that the spiral morphotype actively utilizes lactate while the spherical morphotype does not. Using 13C-labeled lactate as a single carbon source, we investigated the metabolic pathways of lactate utilization in mid-log cells of strain PCS with sulfate as the terminal electron acceptor. The isotopomer analysis of proteinogenic amino acids was performed using both gas chromatography-mass spectrometry and Fourier transform-ion cyclotron resonance mass spectrometry. Based on the labeling pattern of 8 key metabolites alanine, histidine, serine, isoleucine, leucine, aspartate, succinate and glutamate, we observed several unique metabolic pathways in strain PCS. In this organism, a branched tricarboxylic acid cycle exists due to no activity of ketoglutarate dehydrogenase. Also, the lack of an oxidative functional pentose phosphate pathway was observed. The results predict presence of a Re-type citrate synthase, similar to the recently characterized citrate synthase of Clostridia, while isoleucine synthesis seems to be completely via citramalate pathway rather than via L-threonine dehydratase. The isotope labeling pattern of amino acids allowed a preliminary prediction of the in vivo metabolic pathways through central pathways especially useful for microbes whose genetic fingerprint have yet to be completely deciphered.