In an operon, the switch that controls mRNA synthesis is the promotor, which is a sequence of bases upstream from the first structural gene.

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The switch that controls mRNA synthesis in an operon is the operator, not the promoter. The promoter is the site where RNA polymerase binds to initiate transcription, while the operator is the sequence that acts as the regulatory switch, often controlled by a repressor or activator protein.

In an Operon, the Switch That Controls MRNA Synthesis Is