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(d) Lac Repressor and the Lac Operator

One cannot measure an information content from a single sequence. Dyad symmetries in DNA (palindromes) are an exception because both the sequence of the palindrome and its complement are available. This allows us to estimate how much information appears in the lac operator (Beckwith, 1978; Goeddel et al., 1978; Sadler et al., 1983a). Gilbert and Maxam (1973) found that the tetrameric lac repressor protein protects 24 base pairs from DNase digestion. This is a region from -13 to +10, where the zero is the central base. More recently, exonuclease III digestion gave the range -14 to +16 (Shalloway et al., 1980). To analyze the site we extended the range -16 to +16 by 5 bases on both sides (Fig. 5). This range includes the "extended operator" (Dickson et al., 1975; Heyneker et al., 1976). As with other operators, the sequence was compared to its complement using the program Rseq. The central position was included, giving R_sequence = 19.2 bits per site. Because there are only two examples, there is a large sampling error. If there is only one functional lac repressor binding site in the E. coli genome, then R_frequency = 21.9 bits per site. "Pseudo"-operator sequences exist for which there is no known function (Reznikoff et al., 1974; Winter and von Hippel, 1981). If we include the strong secondary "pseudo"-operator, R_sequence = 16.2$\sim$2.6 and R_frequency = 20.9 bits.