FIG 2.

The model of Van Hofvegen et al. proposes adaptation to aerobic utilization of citrate by a multistep process. The E. coli cit operon (citCDEFXGT) encodes a citrate lyase and the citrate permease CitT—a citrate/succinate antiporter. The cit genes are expressed only anaerobically, leaving cells unable to grow aerobically on citrate. Restored expression of the CitT transporter allows E. coli to use citrate as a carbon and energy source (6). During the potentiation step, duplication and amplification of the cit region are proposed to slightly improve growth and amplification of the region provides enough target sequence to permit the rare deletion event that provides a promoter expressing CitT and generates a new shorter nested duplication. This remodeling is the actualization step, which fuses the citT coding sequence to an aerobically expressed gene (such as uspG), allowing aerobic growth on citrate. During the refinement step, growth on citrate may be improved by amplification or increased transcription of the dctA gene encoding an aerobic C4-dicarboxylate transporter (DctA). This transporter improves growth by recapturing the succinate which is lost during uptake of citrate.