When RNA polymerase is bound to the lac promoter sequence, transcription of the lac operon is on.

The statement is not entirely correct. The binding of RNA polymerase to the lac promoter sequence does not necessarily mean that transcription of the lac operon is "on".

Here's the step-by-step process:

1. The lac operon is a set of genes in E. coli that are involved in the metabolism of lactose. It consists of three genes (lacZ, lacY, and lacA) and is controlled by a promoter (lacP) and an operator (lacO).

2. RNA polymerase binds to the promoter sequence, which is upstream of the genes. This is the first step in transcription.

3. However, the lac operon is also regulated by a repressor protein (lacI). In the absence of lactose, the lacI protein binds to the operator sequence, which overlaps with the promoter. This prevents RNA polymerase from transcribing the genes.

4. When lactose is present, it binds to the lacI protein and causes it to change shape and release from the operator. This allows RNA polymerase to transcribe the genes.

5. Therefore, even if RNA polymerase is bound to the lac promoter, transcription of the lac operon will not occur unless lactose is present to inactivate the lacI repressor protein.

So, the presence of RNA polymerase at the lac promoter is necessary but not sufficient for transcription of the lac operon. The operon is only "on" when both RNA polymerase is bound to the promoter and the repressor is inactivated by lactose.