T7 RNA polymerase is a recombinant enzyme purified from Escherichia coli, for the synthesis of high specific-activity RNA probes, biologically active mRNA and antisense RNA. T7 RNA polymerase is DNA-dependent with strict specificity for its own double-stranded promoter that is not efficiently recognized by SP6 or T3 RNA polymerases. T7 RNA polymerase catalyzes the 5’→3’ synthesis of RNA from ribonucleoside triphosphates on single or double-stranded DNA downstream from a T7 promoter. Using circular plasmid DNA as a template will result in heterogeneous transcripts of multiple lengths. T7 RNA polymerase accepts modified nucleotides as substrates for RNA synthesis.