Cloning strategy — Because native Ti plasmids are very large, intermediate (shuttle) vectors were developed by subcloning T-DNA into which commonly used plasmid backbone for <i>E. coli</i> propagation?

Introduction / Context:
Early Ti plasmids were too large and unwieldy for standard cloning in E. coli. The solution was to construct intermediate vectors that carry T-DNA regions within small, well-characterized bacterial plasmids, enabling routine cloning, selection, and amplification before plant transformation via Agrobacterium.

Given Data / Assumptions:

• Goal: manipulate T-DNA cargo in E. coli first, then deliver it into plants.
• pBR322 is a classic, small, multi-copy plasmid with amplicon, ori, and selectable markers.
• pRK2013 is typically a mobilization/helper plasmid, not the standard cloning backbone for subcloning T-DNA cassettes.

Concept / Approach:
Intermediate vectors (and later, binary vectors) utilize pBR322-derived backbones for stability in E. coli. These constructs are then introduced into Agrobacterium along with vir functions (on the same plasmid or a helper), allowing T-DNA transfer to plants.

Step-by-Step Solution:

Verification / Alternative check:
Standard binary vectors (e.g., pBIN19 lineage) trace design features to pBR322-like origins of replication and antibiotic markers for E. coli growth.

Why Other Options Are Wrong:

Common Pitfalls:
Assuming any conjugal helper plasmid is also the cloning vector; roles are distinct in Agrobacterium systems.

Final Answer:
pBR322-based plasmid vector.