At least three operons of Escherichia coli are transcriptionally controlled by the trp aporepressor: the tryptophan biosynthetic operon trpEDCBA, the aromatic amino acid biosynthesis operon aroH and the gene for trp aporepressor itself, trpR (Bennett et al., 1976; Gunsalus and Yanofsky, 1980; Singleton et al., 1980; Bogosian et al., 1981; Zurawski et al., 1981; Joachimiak et al., 1983).
A single dimer of aporepressor binds to the operator in the presence of L-tryptophan (Joachimiak et al., 1983). Likewise, each binding site contains a two-fold symmetry protected by aporepressor from nucleases. We define the center of this symmetry to be between positions 0 and 1 (Fig. 4). A deletion ending at one end of the trp operator, trpLC145, is thought to define the range of the sites, since it does not affect repression (Bertrand et al., 1976; Bennett and Yanofsky, 1978). However, when E. coli trp aporepressor is bound to trp operator DNA of S. typhimurium and the methylation of unprotected purine residues is measured (Oppenheim et al., 1980), the aporepressor protects the region -13 to +14 rather than -11 to +12. We used the range covering 5 bases on either side of this protected area, giving Rsequence = 23.4 bits per site. If one uses the exact range defined by deletion trpLC145, Rsequence would be 20.6.
Although non-physiologically high concentrations of trp aporepressor can regulate several other operons (Johnson and Somerville, 1983; Bogosian and Somerville, 1983), we calculated Rfrequency for only three sites. The relevant genome is that of E. coli, so Rfrequency = 20.3 bits per site.