This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Metabolism of nucleic acids, Subject name: Biochemistry
[{'content': 'CAP in Lac operon is an example of \nSelect and explain. \nA. Positive regulator, B. Negative regulator, C. Constitutive expression, D. Attenuation', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Catabolite activator protein(CAP) must bind to cAMP to activate transcription of the lac operon by RNA polymerase. CAP is a transcriptional activator with a ligand-binding domain at the N-terminus and a DNA-binding domain at the C-terminus.', 'role': 'assistant'}]