Ancient genes in contemporary persistent microbial pathogens.

Autotrophs, the earliest prokaryotes, use CO(2) as the sole or the key source in the reductive citric acid cycle for carbon fixation. This pathway, also known as the reductive tricarboxylic acid (rTCA) cycle, has as its center the Krebs cycle running in the reductive direction, using reduced cofactors for energy. During the infection process, persistent pathogenic bacteria like Mycobacterium tuberculosis, Helicobacter pylori, and Salmonella typhi experience diverse and hostile environments both intracellularly (in macrophages) and extracellularly. M. tuberculosis, for example, must adapt to nutrient-deprived, hypoxic conditions in the granuloma. Genomic annotations reveal the presence of the key enzymes of the rTCA cycle--citrate lyase (Enzyme Commission number EC 4.1.3.6) and 2-oxoglutarate synthase (EC 1.2.7.3)--along with the rest of the TCA cycle enzymes. It is possible that there is a metabolic switch to anaerobic respiration in which a complete or a partial TCA cycle may operate in the reductive mode. This switch would both facilitate carbon fixation and restore the balance of oxidative and reductive reactions during environmental transitions, thus enabling the pathogen to survive, grow, and persist. Verification of enzyme function by biochemical investigations and validation of gene essentiality by knockout studies may reveal these enzymes to be rational drug targets for treatment of persistent microbial infections in mechanism-based drug discovery processes.