SV40 vector systems — requirements for late-region replacement constructs SV40 cloning vectors in which late-region coding sequences are replaced generally require what for productive packaging?

Introduction / Context:
SV40 (Simian Virus 40) has been used historically as a small DNA virus vector. Different designs either delete early or late regions to make room for inserts. Understanding which host or helper functions are required informs how to produce vector stocks and which cells to use.

Given Data / Assumptions:

• Late-region replacement removes capsid protein coding sequences (VP1–VP3).
• Early region (e.g., large T antigen) may be intact or complemented by host cells like COS.
• Packaging of vector genomes into virions requires late gene products.

Concept / Approach:
Late-region replacement vectors lack structural proteins needed for virion assembly. Therefore, a helper SV40 providing the late gene products in trans is required for packaging. By contrast, vectors with early-region deletions can replicate in COS cells expressing large T antigen. Thus, the specific requirement depends on which region is deleted. Here, with late-region replacement, helper virus supply is the key.

Step-by-Step Solution:

Verification / Alternative check:
Classical SV40 vector literature distinguishes early-deletion vectors (require T antigen in COS cells) from late-deletion vectors (require helper for capsid proteins).

Why Other Options Are Wrong:

• COS cells supply T antigen (early function), not late capsid proteins.
• pBR322 is a bacterial plasmid backbone; it does not supply viral proteins.
• Adenovirus E1A is irrelevant to SV40 late gene provision.
• None of these is incorrect because helper is needed.

Common Pitfalls:
Memorizing COS cell requirement and applying it generally; always match the deleted region to the required complementing system.

Final Answer:
SV40 helper virus to supply late gene functions in trans