Which part of an operon acts as the binding site for a repressor protein?

The operator is the segment of the operon that serves as the specific binding site for the repressor protein. In the context of gene regulation in prokaryotes, such as in the lac operon, when a repressor protein binds to the operator, it physically obstructs the progression of RNA polymerase along the DNA, thus preventing the transcription of downstream genes. This binding regulates the gene's expression in response to specific metabolites or environmental signals, allowing bacteria to efficiently turn on or off the genes based on their needs, such as in the presence or absence of lactose.

In this scenario, the promoter is where RNA polymerase binds to initiate transcription, while the gene refers to the specific sequence that encodes for a particular product, such as a protein. The regulator typically refers to a distant site or gene that encodes the repressor protein but is not directly involved in binding to the DNA of the operon itself. Understanding the role of the operator is crucial for grasping how operons function in gene regulation.