Induction occurs when the physiological inducer, allolactose, binds to the lac repressor, preventing it from binding to the operator [6, 8]. Nonphysiological analogs, such as thiogalactosides, can function as inducers too. A single mutation, W220F, in the inducer-binding site of the LacI repressor reduces leakiness [16]. The LacI protein belongs to the GalR/LacI family and as a member of this family of transcriptional regulators, LacI contains three domains: a helix-turn-helix motif located in the N terminus, the central domain that binds to LacI sugar ligands, and the C terminal, which comprises the tetramerization domain [6]. The crystal structure of the LacI binding site for allosteric effectors has been resolved [1]. The importance of the N125 and D149 residues of the N terminal in the recognition of the sugar effector binding site has been determined [1, 17]. Based on a computational rod model of the DNA-LacI complex, the looping of both linear DNA and supercoiled DNA minicircles over a broad range of DNA interoperator lengths was analyzed. Analysis showed that the most stable loops for linear DNA occurred when LacI adopted the extended conformation [18]. A computational analysis suggested that the hinge region of LacI is important for the nonspecific binding to DNA, where the electrostatic interactions among protein, DNA, and salt ions are crucial [19]. Using a method adapted to high-resolution mapping of protein binding to DNA in living cells, binding of three different DNA loop sizes with the Nhp6A architectural protein was detected for a single sequence in the lac promoter [20] . On the other hand, in an in vivo single-molecule assay, the sliding process for the LacI TF was determined. LacI slides 45 +/- 10 bp on chromosomal DNA, and this can be obstructed by other DNA-bound proteins near the operator. LacI binds to the O1, O2, and O3 operators upstream of the lacZ promoter [21]. It slides over its natural lacO1 operator several times before binding ARRAY(0x2872e68) , suggesting a trade-off between a rapid search on nonspecific sequences and fast binding at the specific sequence [22]. A 91-bp LacI-mediated, negatively supercoiled DNA loop mimics the DNA loop between the O1 and O3 operators in the lacZp1 promoter [23] . The stability of DNA topological barriers generated by LacI is proportional to its DNA binding afinity [24]. DNA supercoiling is capable of regulating the basal expression of the lac operon [24]."