The protein that inhibits transcription of lac operon is tetramer having four identical subunits that are called lac repressor.
In lac operon, lactose acts as inducer and if lactose is provided in the medium for the bacteria then the regulatory gene is activated. The inducer binds to the repressor protein and renders it inactive which allows transcription of the operon. Therefore, the lac operon is negatively regulated in this case.
As one gene product controls the expression of multiple operons this is known as global control. Global positive control is the common form of gene regulation in eukaryotes. Global negative control occurs commonly, but specific negative control in the lac operon is very rare.
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