What is the name of the protein that controls transcription of an operon by binding to the operator?

The correct answer is the repressor, which is a specific type of protein that plays a crucial role in gene regulation within operons. Repressors work by binding to a specific DNA sequence known as the operator, located within the operon. When the repressor is bound to the operator, it physically blocks RNA polymerase from transcribing the downstream genes, effectively inhibiting their expression. This action allows the cell to regulate gene expression in response to environmental conditions, ensuring that proteins are produced only when they are needed.

In contrast, the other choices represent different kinds of proteins involved in gene regulation but function differently. Activators enhance the transcription of genes by promoting the binding of RNA polymerase to the promoter region. Enhancers are distant regulatory DNA sequences that can increase transcription levels but do not directly bind to the operator or act to inhibit transcription. Transcription factors are a broad category of proteins that assist in the process of transcription but are not specifically defined by their repressive function. Thus, the repressor is uniquely important for its role in inhibiting transcription by binding to the operator within the operon.