T7 RNA polymerase

T7 Polymerase is an RNA polymerase that catalyzes the formation of RNA in the 5'→ 3' direction. T7 polymerase is extremely promoter-specific and only transcribes bacteriophage T7 DNA or DNA cloned downstream of a T7 promoter. The T7 polymerase also requires a DNA template and Mg2+ ion as cofactor for the synthesis of RNA. The enzyme is strongly stimulated by BSA or spermidine. In contrast to bacterial RNA polymerases, T7 polymerase is not inhibited by the antibiotic rifampicin. The source of the enzyme is the T7 bacteriophage, which is a virus that infects only bacteria. This phage's polymerase has a very low error rate. T7 polymerase has a molecular weight of 99 kDa.

T7 RNA polymerase is commonly used to transcribe DNA which has been cloned into vectors which have two phage promoters in opposite orientation. RNA can be selectively synthesized from either strand of the insert DNA with different polymerases.

Homogeneously labeled single-stranded RNA can be generated with this system. Transcripts can be non-radioactively labeled to high specific activity with certain labeled nucleotides.