Identification of succinate exporter in Corynebacterium glutamicum and its physiological roles under anaerobic conditions

Corynebacterium glutamicum produces succinate from glucose via the reductive tricarboxylic acid cycle under microaerobic and anaerobic conditions. We identified a NCgl2130 gene of C. glutamicum as a novel succinate exporter that functions in succinate production, and designated sucE1. sucE1 expression levels were higher under microaerobic conditions than aerobic conditions, and overexpression or disruption of sucE1 respectively increased or decreased succinate productivity during fermentation. Under microaerobic conditions, the sucE1 disruptant sucE1Δ showed 30% less succinate productivity and a lower sugar-consumption rate than the parental strain. Under anaerobic conditions, succinate production by sucE1Δ ceased. The intracellular succinate and fructose-1,6-bisphosphate levels of sucE1Δ under microaerobic conditions were respectively 1.7-fold and 1.6-fold higher than those of the parental strain, suggesting that loss of SucE1 function caused a failure of succinate removal from the cells, leading to intracellular accumulation that inhibited upstream sugar metabolism. Homology and transmembrane helix searches identified SucE1 as a membrane protein belonging to the aspartate:alanine exchanger (AAE) family. Partially purified 6x-histidine-tagged SucE1 (SucE1-His]₆) reconstituted in succinate-loaded liposomes clearly demonstrated counterflow and self-exchange activities for succinate. Together, these findings suggest that sucE1 encodes a novel succinate exporter that is induced under microaerobic conditions, and is important for succinate production under both microaerobic and anaerobic conditions.