Final answer:
The operator's primary function is to act as a binding site for a repressor protein, which prevents transcription by blocking RNA polymerase in bacterial cells.
Step-by-step explanation:
The primary function of an operator in the regulation of transcription in bacteria is to serve as a binding site for a repressor protein. This repressor can physically block RNA polymerase from transcribing the structural genes of an operon. Operons, like the lac and trp operons, are regions of DNA that consist of genes encoding proteins needed for a specific function, and are regulated through the interplay of activators, repressors, and inducers
The repressor, by binding to the operator, prevents transcription in response to an external stimulus. Conversely, an activator enhances transcription by facilitating RNA polymerase binding to the promoter, and inducers can either deactivate a repressor or help an activator, depending on the operon's needs.