Fig. 1
a Schematic diagram for construction of the synthetic nar promoter library. The 15-bp spacer sequence (indicated by consecutive 15 Ns) between the − 35 and − 10 elements of the wild-type nar promoter was randomized. An FNR binding site is shown in italics; the − 35 and − 10 elements are indicated by underlines; a red arrow indicates a transcription start site (+ 1); SD, Shine–Dalgarno sequence, GFPm; modified green fluorescence protein with His6 tag. b Reconstructed d-lactate and 2,3-butanediol (2,3-BDO) biosynthesis pathways in E. coli. The d-lactate pathway (red) consists of ldhD gene encoding d-lactate dehydrogenase (LDHD); the 2,3-BDO pathway (blue) consists of ilvBN gene encoding acetohydroxybutanoate synthase/acetolactate synthase (IlvBN), aldB gene encoding acetolactate decarboxylase (AldB), and bdh1 encoding butanediol dehydrogenase (BDH1)