Final answer:
Lambda repressors C1 and Cro regulate gene expression in the lambda phage by binding to operators and affecting RNA polymerase's ability to initiate transcription, not directly on replication enzymes like DNA polymerase, helicase, or DNA ligase.
Step-by-step explanation:
Lambda repressors C1 and Cro act on the bacterial lambda phage by regulating its gene expression. They achieve this by binding to specific DNA sites called operators, which in turn affects RNA polymerase. The repressor binds to the promoter, which is the DNA sequence where RNA polymerase normally attaches to start transcription. When the lambda repressor C1 is bound to the operator, it prevents RNA polymerase from initiating transcription, thus maintaining the lambda phage in a lysogenic state. Conversely, when Cro is bound to the operator, it prevents the lambda repressor C1 from maintaining the lysogenic state, leading to the lytic cycle activation where the phage DNA is replicated.
These repressors do not act directly on any of the replication enzymes, such as DNA polymerase, helicase, DNA ligase, or primase. Therefore, the correct answer to what lambda repressors C1 and Cro act on is:
- b. the repressor binds to the promoter