Fig. 4

Correlation between 1-butanol productivity and enzyme expression by the different FRE. a Comparison of enzyme expressions in the low and high producers of 1-butanol. Two low and two high producers (including the best producer) from the screening experiment were used in this study. The best 1-butanol producer (FRE _adhE ::fdh, FRE _ackA ::atoB-adhE2-crt-hbd, and FRE _adhE ::ter) is shaded with dark gray in the table below, while the one particular FRE change in the other three variants is shaded in light gray. Strain ΔldhA ΔfrdBC ΔadhE Δpta was used as the host. Identical cultivation procedure for 1-butanol production as in the screening process was followed. Samples were taken and cells were harvested for in vitro assay after 24 h of anaerobic switch. BDH, AdhE2 activity toward butyraldehyde; BYDH, AdhE2 activity toward butyryl-CoA. b Time course of Fdh expression regulated by each FRE. The highest 1-butanol producer (FRE _ackA ::atoB-adhE2-crt-hbd, FRE _adhE ::ter, FRE _adhE ::fdh) along with its three other FRE::fdh variants were used in this study. All strains contained FRE _ackA ::atoB-adhE2-crt-hbd and FRE _adhE ::ter but different FRE::fdh as shown in the figure legend. Strain ΔldhA ΔfrdBC ΔadhE Δpta was used as the host. Identical cultivation procedure for 1-butanol production as in the screening process was followed except that cells were switched to anaerobic condition around an OD₆₀₀ of 0.4. Samples were taken at various time points and cells were harvested for in vitro assay of Fdh activity. “Time” indicates time since anaerobic switch