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The lac repressor is a DNA-binding protein which inhibits the expression of genes coding for proteins involved in the metabolism of lactose in bacteria. This occurs in the absence of lactose, presumably so that the organism doesn't waste amino acids producing unnecessary proteins. When lactose becomes available, it is converted into allolactose, which inhibits the Lac repressor's DNA binding ability.
Function
The lac repressor (LacI) operates by binding to the operator region of the lac operon. This blocks RNA polymerase from binding, and so prevents transcription of the mRNA coding for the lac proteins. When lactose is present, allolactose binds to the lac repressor, causing an allosteric change in its shape. In its changed state, the lac repressor is unable to bind to the lac operator.
Structure
The lac repressor protein has three distinct regions:
- a core region (which binds allolactose)
- a tetramerization region (which joins four monomers in an alpha-helix bundle)
- a DNA-binding region (in which two LacI proteins bind a single operator site)
The lac repressor occurs as a tetramer (four identical subunits bound together). This can be viewed as two dimers, with each dimer being able to bind to a single lac operator. The two subunits each bind to a slightly separated (major groove) region of the operator. The promoter is slightly covered by the lac repressor so RNAP cannot bind to and transcribe the operon.
The DNA binding region consists of a helix-turn-helix structural motif.
Discovery
The lac repressor was first isolated by Walter Gilbert and Benno Müller-Hill in 1966. They were able to show, in vitro, that the protein bound to DNA containing the lac operon, and released the DNA when IPTG was added. (IPTG is an allolactose analog.) They were also able to isolate the portion of DNA bound by the protein by using the enzyme deoxyribonuclease, which breaks down DNA. After treatment of the repressor-DNA complex, some DNA remained, suggesting that it had been masked by the repressor. This was later confirmed.
These experiments were important, as they confirmed the mechanism of the lac operon, earlier proposed by Jacques Monod and Francois Jacob.
See also
"Lac repressor"