Distribution of promoter architectures in E. coli. We considered the binding sites in E. coli K-12, which were listed in RegulonDB (58). (A–C) plot histograms of the overlap or the distance between two sites that form a (A) switch, (B) barrier and (C) cluster. Next, we counted the number of (D) switches, (E) barriers and (F) clusters and found that these building blocks are frequently encountered in E. coli K-12 genome. For the barrier and cluster pairs, we consider binding sites that are <10 bp apart. In (G) and (H), we presented some examples of complex promoters: (G) double barrier and (H) double barrier cluster.