Binary vectors

A transfer DNA (T-DNA) binary system is a pair of plasmids consisting of a binary plasmid and a helper plasmid. The two plasmids are used together (thus binary) to produce genetically modified plants. They are artificial vectors that have both been created from the naturally occurring Ti plasmid found in Agrobacterium tumefaciens. The binary vector is a shuttle vector, so-called because it is able to replicate in multiple hosts (E. coli and Agrobacterium tumefaciens).

Systems in which T-DNA and vir genes are located on separate replicons are called T-DNA binary systems. T-DNA is located on the binary vector (the non-T-DNA region of this vector containing origin(s) of replication that could function both in E. coli and in Agrobacterium tumefaciens, and antibiotic-resistance genes used to select for the presence of the binary vector in bacteria, became known as vector backbone sequences). The replicon containing the vir genes became known as the vir helper. Strains harboring this replicon and a T-DNA are considered disarmed if they do not contain oncogenes that could be transferred to a plant.

There are several binary vector systems that differ mainly in the plasmid region that facilitates replication in Agrobacterium. Commonly used binary vectors include:

Common Helper plasmids include

The T-DNA portion of the binary plasmid is flanked by left and right border sequences and consists of a transgene as well as a plant selectable marker (PSM). Outside of the T-DNA, the binary plasmid also contains a bacterial selectable marker (BSM) and an origin of replication (ori) for bacteria.

The helper plasmid contains the vir genes that originated from the Ti plasmid of Agrobacterium. These genes code for a series of proteins that cut the binary plasmid at the left and right border sequences, and facilitate transduction of the T-DNA to the host plant's cells. The helper plasmid also contains a BSM and an ori for bacteria.

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