lac repressor
Summary
The lac repressor is the E. coli protein that binds the lac operator and blocks transcription of the lactose-use genes. Allolactose, made from lactose, binds it and releases it from DNA, so the operon is inducible. Mutants that cannot bind DNA give constitutive expression.
Detail
Meaning: The lac repressor (LacI) is an E. coli DNA-binding protein that binds the operator of the lac operon and blocks RNA polymerase from transcribing lacZ, lacY and lacA, the genes for lactose use. Allolactose, an isomer made from lactose, binds the repressor and changes its shape, so it releases the DNA and transcription can proceed. This is negative, inducible control.
On the MCAT: Passages describe mutants and ask whether β-galactosidase is made with or without lactose and glucose. A lacI mutant that cannot bind DNA gives constitutive expression. A mutant repressor that cannot bind allolactose stays on the operator and keeps the operon off even with lactose. An operator mutation that blocks repressor binding also gives constitutive expression, but only for genes on the same DNA (cis), while the repressor protein diffuses and acts in trans. High expression also needs low glucose, so CAP–cAMP can bind.
Common confusion: Students often say lactose itself binds the repressor. The inducer is allolactose, and it inactivates the repressor rather than activating it. Releasing the repressor also does not guarantee high transcription: with high glucose, cAMP and therefore CAP–cAMP are low, so output stays low.
Sources
- Lehninger Principles of Biochemistry
- Biochemistry (Berg, Tymoczko, Gatto, Stryer)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.