A high concentration DNA-dependent RNA Polymerase with very high specificity for the T7 phage double-stranded promoter sequence. The enzyme catalyzes the 5’-3’ synthesis of RNA on either single-stranded DNA or double-stranded DNA downstream from the promoter and is the industry standard enzyme for in vitro transcription (IVT). It can efficiently incorporate standard or modified ribonucleoside triphosphates (NTPs) synthesizing large amounts of mRNA very quickly, making it an ideal enzyme for in vitro synthesis of labeled or unlabeled RNA for research, diagnostic or clinical applications. Applications including isothermal diagnostic methods, such as nucleic acid sequence-based amplification (NASBA), mRNA vaccine production, and RNA-based therapeutic and research applications, including miRNA and siRNA synthesis for RNA interference drugs, synthesis of sgRNA for the CRISPR/Cas9 system.