SV40 early-region replacement vectors — required complement for growth SV40-based vectors in which the early region is replaced by foreign DNA will replicate/express efficiently only if which support is provided?

Introduction / Context:
SV40 vectors exploit the SV40 origin of replication and early/late gene organization. When the early region (coding for large T antigen) is replaced, the vector loses essential functions needed for replication and must be complemented.

Given Data / Assumptions:

• SV40 early region encodes large T antigen required for initiating replication at the SV40 origin.
• COS cells constitutively express SV40 large T antigen.
• The question asks which support rescues vectors with early-region replacement.

Concept / Approach:

Large T antigen binds the SV40 ori, unwinds DNA, and recruits host replication machinery. Early-region replacement vectors therefore depend on a cell line that supplies large T in trans. COS cells are designed for precisely this purpose, enabling robust replication and expression of SV40-ori plasmids.

Step-by-Step Solution:

Verification / Alternative check:

Protocols for SV40 vectors consistently recommend COS-1/COS-7 cells to complement early-region defects, confirming this requirement.

Why Other Options Are Wrong:

Helper viruses providing late genes (option b) do not restore early functions; pBR322 (option c) is bacterial; baculovirus systems (option e) are insect-specific; early region is not dispensable (option d).

Common Pitfalls:

Assuming any SV40-related product suffices; only large T antigen complementation enables ori-driven replication.

Final Answer:

COS cell line supplying SV40 large T antigen in trans