Carbon fluxes rewiring in engineered E. coli via reverse tricarboxylic acid cycle pathway under chemolithotrophic condition.

BACKGROUND

A transgenic strain of Escherichia coli has been engineered to directly assimilate gaseous CO into its biomass through hydrogen-powered anaerobic respiration. This was achieved by expressing key components of the reverse tricarboxylic acid (rTCA) cycle, including genes encoding α-ketoglutarate: ferredoxin oxidoreductase (KOR) and ATP-dependent citrate lyase (ACL) from Chlorobium tepidum. These enzymes were selected for their essential roles in enabling CO fixation and integration into central metabolism.

RESULTS

This study found that KOR alone can support cellular maintenance under chemolithotrophic conditions, while additional expression of ACL enhances CO assimilation. Using isotopic CO tracing, it was demonstrated that KOR alone facilitates CO assimilation into TCA metabolites. However, co-expression of ACL with KOR redirected carbon fluxes from TCA cycle toward essential metabolic pathways, particularly those involved in protein and nucleotide biosynthesis. Compared to KOR alone, ACL co-expression significantly increased isotopic enrichments in amino acids (e.g., methionine, threonine, glycine) and nucleotides (e.g., deoxythymidine, deoxycytidine). These results suggest that ACL supports the synthesis of nitrogen-containing metabolites when inorganic nitrogen is sufficient, while KOR alone sustains core metabolic functions under chemolithotrophic conditions.

CONCLUSIONS

This study demonstrates a novel strategy to engineer E. coli for CO fixation using only one or two heterologous enzymes under chemolithotrophic conditions. These findings reveal the minimal genetic and nutritional requirements for CO assimilation and provide insights into metabolic flux partitioning in engineered strains. This research paves the way for sustainable applications in carbon fixation and biotechnological innovation.