A primordial and reversible TCA cycle in a facultatively chemolithoautotrophic thermophile

Abstract

About-face for citrate synthase Classically, it is thought that citrate synthase only works in one direction: to catalyze the production of citrate from acetyl coenzyme A and oxaloacetate in the tricarboxylic acid (TCA) cycle. The TCA cycle can run in reverse to cleave citrate and fix carbon dioxide autotrophically, but this was thought to occur only with alternative enzymes, such as citrate lyase. Now Nunoura et al. and Mall et al. have discovered thermophilic bacteria with highly efficient and reversible citrate synthase that requires reduced ferredoxin (see the Perspective by Ragsdale). This function is undetectable by metagenomics, but classical biochemistry filled in the gaps seen between the genome sequences and the phenotypes of the organisms. The direction of catalysis depends on the availability of organic versus inorganic carbon and reflects a flexible bet-hedging strategy for survival in fluctuating environments. In evolutionary terms, this capacity might predate the classical TCA cycle and is likely to occur in a wide range of anaerobic microorganisms. Science , this issue p. 559 , p. 563 ; see also p. 517