T7 RNA polymerase is a DNA-dependent RNA polymerase that specifically recognizes the T7 promoter sequence (5' -TaatacgactCactatagggaga-3 ') and has high specificity for the phage T7 promoter sequence. T7 RNA polymerase can catalyze NTP incorporation of single or double stranded DNA template downstream of T7 promoter to synthesize RNA complementary to the downstream DNA template of T7 promoter.