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.
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