Promoter activities per cell under non-induced (0 mM IPTG) and induced (1 mM IPTG) conditions, characterized using an EYFP reporter construct carried on pPMQAK1 in Escherichia coli NEB5α F’ Iq(New England Biolabs). A. Control promoters. J23101 corresponds to BioBrick BBa_J23101 (iGEM Registry ) and is a constitutive control promoter without lac operator (lacO). Ptrc 2O-original is the Ptrc 2O version previously characterized  with lacO1 in the proximal position and the symmetric lacO (lacO sym) in the distal position. Ptrc 1O-distal, -core, and-proximal are Ptrc-derived promoters with only one lacO sym at the indicated position. PA1lacO-1 is a previously used promoter with lacO in the core and proximal positions . B. The Ptrc 2O-library. The length of the spacer sequence between the −35 box and the distal lacO sym is indicated on the upper x-axis and the distance between the two lacO sym is indicated on the lower x-axis. Promoter activities per cell are given in fluorescence/absorbance at 595 nm in arbitrary units [AU]. Error bars indicate standard error of the mean (n = 6). The numbers on top of the columns are the induction ratios. na, not available (repressed fluorescence under detection limit). IPTG, Isopropyl β-D-1-thiogalactopyranoside.