Such a mechanism could account for regulation of DNA looping in other systems. Slight reorientation of the subunits of AraC could specify looping or unlooping by the protein. The conversion between the two states does not require additional binding of AraC protein and appears to be driven largely by properties of the protein rather than being specified by the slightly different DNA sequences of the binding sites. 14 The main operator (O1) in the lac operon is located slightly downstream of the promoter two additional operators, O2 and O3 are located at -82 and +412, respectively. DnaA protein performs a number of essential functions in initiation, including recognition of each DnaA box sequence in oriC.DNA binding assays first showed that DnaA specifically recognizes oriC as well as to other DNAs carrying the DnaA box sequence. It is classically defined in the lac operon as a segment between the promoter and the genes of the operon. Kaguni, in Encyclopedia of Biological Chemistry, 2004 DnaA Protein and the DnaA Box. The addition of arabinose, which induces the operon, breaks the loop, and shifts the interactions from the distal araO 2 site to the previously unoccupied half of the araI site. Operator a segment of DNA to which a repressor binds. In vivo and in vitro experiments presented here show that an AraC dimer, with binding to half of araI and to araO 2, maintains the repressed state of the operon. The repression of the araBAD promoter is mediated by DNA looping between AraC protein bound at two sites near the promoter separated by 210 base pairs, araI and araO 2. The replication initiator factor RepE of the F plasmid in Escherichia. Expression of the L-arabinose BAD operon in Escherichia coli is regulated by AraC protein which acts both positively in the presence of arabinose to induce transcription and negatively in the absence of arabinose to repress transcription. and preliminary X-ray crystallographic study of its complex with operator DNA.