Baculovirus expression system — Where is a foreign gene positioned to achieve very high-level expression in insect cells?

Introduction / Context:
Baculovirus expression vectors (BEVs), especially those based on Autographa californica multiple nucleopolyhedrovirus (AcMNPV), are widely used to produce recombinant proteins in insect cells. The hallmark of the system is the use of extremely strong late promoters to drive high-level expression of a heterologous gene.

Given Data / Assumptions:

• Polyhedrin (polh) and p10 are strong late promoters in AcMNPV.
• Foreign open reading frames can replace native structural genes to co-opt the strong promoter.
• High expression requires correct promoter–ORF orientation and proper transcription/termination elements.

Concept / Approach:
Replacing the polyhedrin coding region with the gene of interest positions the heterologous gene immediately downstream of the polyhedrin promoter. This harnesses the natural late-phase transcriptional surge, often yielding very high protein levels suitable for structural biology and vaccine production.

Step-by-Step Solution:

Verification / Alternative check:
Commercial BEV kits and protocols map insertion at the polyhedrin locus; titers and expression levels confirm this strategy.

Why Other Options Are Wrong:

• b: Origins of replication do not drive transcription.
• c: Random insertion leads to unpredictable expression.
• d/e: Suboptimal or incorrect promoter context reduces expression dramatically.

Common Pitfalls:
Confusing replication control with transcriptional control; neglecting late-promoter timing for maximal yield.

Final Answer:
Inserted downstream of the strong polyhedrin promoter (replacing the polyhedrin coding region)