Expanding the scope of redox-balance growth coupling techniques with a carbon co-feeding strategy

Metabolic engineering to produce molecules not naturally synthesized by the host often requires directed evolution to improve pathway enzyme performance. Growth-coupled selection can dramatically increase directed-evolution throughput, and manipulation of redox balance has proven effective for tying reductase fitness to microbial growth. However, most redox-balance selections require feeding the reductase substrate because of stoichiometric constraints. This is impractical for many biosynthetic pathways either due to practical limitations on cost or complexity of bulk substrate synthesis, or the lack of an ability to transport substrate into cells, for example intracellular acyl-CoA/ACP intermediates. Here we define stoichiometric constraints that make substrate feeding necessary for many acetyl-CoA-derived reduction pathways in NADPH-imbalanced hosts. We overcome these constraints with a dual-feedstock strategy in which glucose provides reducing power while acetate supplies additional acetyl-CoA without directly perturbing redox balance. In an engineered Escherichia coli selection strain, acetate co-feeding enabled growth coupling of acetaldehyde, 3-hydroxybutyrate, and mevalonate production and produced a linear correlation between product formation and growth. We then used this selection to evolve a class II HMG-CoA reductase (HMGR) from Delftia acidovorans toward NADPH utilization, enriching variants with improved NADPH-dependent activity. Finally, propionate co-feeding enabled growth coupling of propionyl-CoA reduction, supporting the generality of carbon co-feeding for selecting enzymes in pathways involving acyl-chain elongation and reduction.