Kinetics of promoter binding: The bacterial closed complex (RNA polymerase bound to promoter DNA before strand opening) is best described as

Introduction / Context:
Transcription initiation involves a sequence of states: free RNA polymerase and promoter, a reversible closed complex, then an isomerization to a more stable open complex with locally melted DNA.

Given Data / Assumptions:

• Closed complex forms by initial binding and can dissociate.
• Open complex formation is promoted by favorable promoter sequences and activators.

Concept / Approach:
The closed complex is a reversible association governed by binding equilibria. Transition to the open complex depends on promoter sequence determinants and regulatory proteins; repressors and activators alter occupancy and isomerization rates.

Step-by-Step Solution:
Define closed complex → pre-melted, reversible binding of polymerase to promoter. Assess statements: equilibrium with free states → correct. Reject claims that promoter mutations or regulators have no effect; they do. Reject irreversibility; closed complex can dissociate without initiation.

Verification / Alternative check:
Kinetic studies show on/off rates for closed complex formation; footprinting and promoter mutations alter binding affinity and open-complex formation.

Why Other Options Are Wrong:
Promoter mutations in −35/−10 boxes greatly affect binding; repressors generally reduce occupancy; activators enhance binding/isomerization; the closed complex is not immediately irreversible.

Common Pitfalls:
Confusing closed with open complex; assuming initial binding guarantees initiation.

Final Answer:
In equilibrium with free RNA polymerase and the promoter.