So. What we have here are various levels of regulation of the lac gene. We have a motor (polymerase), a brake (lac repressor) and an accelerator (CAP). Together with glucose and lactose, they form a tightly regulated system, a cybernetic system that governs its own activity in such a way as to adapt itself to changing circumstances in the environment. It’s not important that you remember the details of the lac operon—after all, you forgot them once before, and you’ve done okay, right? What your Old Quantum Butcher wants you to take away from this is the idea that genetic expression is largely regulated through control of transcription, and control of transcription is largely mediated by DNA binding proteins.