In virology, which virus is known to “rescue” (i.e., enable productive replication of) adenovirus in simian cells by providing missing helper functions?

Introduction / Context:
This question probes your understanding of helper functions and “rescue” phenomena in virology. Certain viruses can complement missing functions of another virus within a specific host cell, permitting productive replication. In simian (monkey) cells, simian virus 40 (SV40) is historically noted for its ability to assist adenovirus replication by providing helper activities the adenovirus lacks in that cellular context.

Given Data / Assumptions:

• Adenovirus requires specific host and viral factors for efficient replication.
• Simian cells may not naturally supply all factors for adenovirus to complete its cycle.
• SV40 expresses early proteins (notably large T antigen) that can modulate host replication machinery.

Concept / Approach:
“Rescue” here means that coinfection or prior infection with a helper virus supplies functions enabling another virus to replicate. SV40 large T antigen drives the host into S phase–like conditions, activates or sequesters key cell-cycle regulators, and can enhance DNA replication environments. Historically, this complementation permits adenovirus to produce progeny in simian cells where it otherwise would be restricted.

Step-by-Step Solution:

Verification / Alternative check:
Classic virology literature and vector systems reference SV40 “helper” effects in primate cell lines, contrasted with other viruses that lack such complementation in this setting.

Why Other Options Are Wrong:

• Vaccinia: a poxvirus with cytoplasmic replication; it does not provide the nuclear helper functions required for adenovirus in simian cells.
• Cytomegalovirus: human betaherpesvirus; its helper roles are host-specific and not known to rescue adenovirus in simian cells.
• Rabies: a negative-sense RNA virus without relevant DNA-replication helper activities.

Common Pitfalls:
Assuming any co-infecting DNA virus will rescue another; helper functions are specific to replication requirements and host context.

Final Answer:
Simian virus 40