Final answer:
The repressor protein is responsible for turning off the expression of the lac operon genes by binding to the operator sequence and preventing RNA polymerase from transcribing the operon when lactose is absent.
Step-by-step explanation:
The protein responsible for turning off the expression of the lac operon genes is the repressor protein. It is produced by the I gene, which is constantly expressed in E. coli cells. When lactose is absent, the repressor protein binds to the operator sequence, blocking the forward movement of RNA polymerase and thereby preventing transcription of the operon. On the other hand, when lactose is present, allolactose (a lactose derivative) binds to the repressor, causing it to release from the operator. This allows RNA polymerase to transcribe the lac operon genes, enabling the metabolism of lactose.