Lac Repressor - Structure

Structure

Structurally, the lac repressor protein is a homo-tetramer. The tetramer contains two DNA binding subunits composed of two monomers each (sometimes called "dimeric lac repressor"). These subunits dimerize to form a tetramer capable of binding two operator sequences. 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 O₁, O₂ and O₃). The O₁ operator sequence slightly overlaps with the promoter, which is thought to therefore prohibit binding by RNA polymerase thereby inhibiting expression of the operon. Additionally, because each tetramer contains two DNA-binding subunits, binding of multiple operator sequences by a single tetramer induces DNA looping.