Structurally, the lac repressor protein is a homotetramer. More precisely, the tetramer contains two DNA-binding subunits composed of two monomers each (a dimer of dimers). Each monomer consists of four distinct regions:
*An N-terminal DNA-binding domain (in which two LacI proteins bind a single operator site)
*A regulatory domain (sometimes called the core domain, which binds allolactose, an allosteric effector molecule)
*A linker that connects the DNA-binding domain with the core domain (sometimes called the hinge helix, which is important for allosteric communication)
*A C-terminal tetramerization region (which joins four monomers in an alpha-helix bundle)
DNA binding occurs via an N-terminal helix-turn-helix structural motif and is targeted to one of several operator DNA sequences (known as O, O and O).  The O operator sequence slightly overlaps with the promoter, which increases the affinity of RNA polymerase for the promoter sequence such that it cannot enter elongation and remains in abortive initiation.  Additionally, because each tetramer contains two DNA-binding subunits, binding of multiple operator sequences by a single tetramer induces DNA looping.