P trc -library promoter design overview and sequences. 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 a proximal position and the symmetric lacO (lacO sym) in a distal position. Ptrc 1O-distal, -core, and-proximal are Ptrc-derived promoters with only one lacO sym in the indicated positions. PA1lacO-1 is a previously used promoter with lacO in the core and proximal positions . B. Overview of the Ptrc 2O-library. The spacer sequence between the −35 box and the distal lacO sym varies in length from −2 bp (where 2 bp of lacO sym is part of the −35 box, as illustrated) to 21 bp. The variable-length spacer sequence is derived from the Ptrc 2O-original spacer sequence. Transcriptional start sites are marked in bold italic.