Vector strategy — In co-integrating transformation systems used with Agrobacterium, where must a region of DNA homology be present to allow recombination?

Introduction / Context:
Co-integrating vector systems predated today’s binary vectors. They rely on homologous recombination between an E. coli-derived vector and the resident Ti plasmid in Agrobacterium, forming a larger co-integrate that carries the engineered T-DNA for plant transformation.

Given Data / Assumptions:

• Homology is required for stable recombination events.
• The Ti plasmid provides virulence functions needed for T-DNA processing and transfer.
• The engineered segment must be captured into a transferable context.

Concept / Approach:
To form a co-integrate, the small cloning vector and the Ti plasmid share a homologous region. Recombination integrates the engineered T-DNA into the Ti backbone inside Agrobacterium, enabling subsequent mobilization of T-DNA into plant cells.

Step-by-Step Solution:

Verification / Alternative check:
Historic co-integrative systems (e.g., pRK290 derivatives) used defined homology regions to capture engineered inserts into Ti backbones.

Why Other Options Are Wrong:

• Homology restricted to only one plasmid cannot mediate inter-molecular recombination.
• “None of these” ignores the fundamental requirement of shared sequence.
• Plant genome homology is irrelevant for plasmid–plasmid recombination in Agrobacterium.

Common Pitfalls:
Confusing co-integrative vectors with binary vectors; binary systems do not require recombination with the Ti plasmid because vir genes act in trans.

Final Answer:
between vector plasmid and Ti-plasmid