At the E. coli oriC, the initial melting of about 40–45 base pairs requires which minimal set of factors?

Introduction / Context:Initiation at the E. coli origin (oriC) proceeds through ordered steps: origin recognition, DNA unwinding (melting), helicase loading, and replisome assembly. The question asks for the minimal factors needed for the initial 40–45 bp origin melting.

Given Data / Assumptions:

• Organism: E. coli, origin oriC.
• Proteins: DnaA (initiator), DnaB (helicase), HU (DNA-bending protein).
• ATP is available for DnaA activity.

Concept / Approach:DnaA-ATP binds DnaA boxes within oriC, oligomerizes, and induces local strand opening in the AT-rich DNA unwinding element (DUE). HU facilitates bending and melting. DnaB helicase acts after melting, being loaded by DnaC.

Step-by-Step Solution:1) DnaA in the ATP-bound state recognizes oriC and multimerizes.2) HU stabilizes wrapped/bent DNA, lowering the energy barrier for unwinding.3) This DnaA-ATP + HU complex opens ~40–45 bp at the DUE.4) Only after opening is DnaB loaded (by DnaC) to expand the replication forks.

Verification / Alternative check:In vitro reconstitution shows origin opening by DnaA-ATP with architectural proteins like HU; helicase loading is a subsequent step, not a prerequisite for initial melting.

Why Other Options Are Wrong:Options lacking DnaA miss the initiator. Options including DnaB or polymerase III add factors needed after melting, not for the minimal opening step.

Common Pitfalls:Equating “melting” with full fork establishment. Helicase action extends unwinding but is not required for the initial 40–45 bp opening.

Final Answer:DNA + ATP + HU + DnaA