Final answer:
The repressor protein of the ARG operon binds to the operator and shuts down transcription when arginine is present as it acts as a co-repressor.
Step-by-step explanation:
The status of the repressor protein and the co-repressor for the ARG operon depends on the presence or absence of arginine. When arginine is present, it acts as a co-repressor that binds to the repressor protein. This complex can then bind to the operator site of the ARG operon, preventing RNA polymerase from transcribing it, effectively turning off the genes involved in arginine synthesis. On the other hand, when arginine is absent, the repressor protein is unable to bind to the operator site without the co-repressor.
Hence, the operon is active, and the genes for arginine synthesis are expressed to produce the necessary enzymes for arginine synthesis.