The T7 DNA polymerase of the T7 bacteriophage is a DNA-dependent DNA polymerase responsible for the fast rate of T7 phage DNA replication in vivo. The polymerase consists of a 1:1 complex of the viral T7 gene 5 protein (80kDA) and the E. coli thioredoxin (12kDA).

It lacks a 5' -> 3' exonuclease domain, but the 3' -> 5' exonuclease activities are approximately 1000-fold greater than that of Klenow fragment. ^[1] The exonuclease activity appears to be responsible for the high fidelity of this enzyme and prevents strand displacement synthesis ^[2]

This polymerase is unique due to its considerable processivity, or ability to stay on DNA for a greater than average number of base pairs. This makes it particularly useful for recombinant protein expression systems; Using the T7 promoter and T7 polymerase strongly drive the inserted gene of interest without inducing host protein overexpression. It is also suitable for site-directed mutagenesis ^[3] but is not recommended for DNA sequencing applications.

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