T7 RNA Polymerase

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There are two broad classes of DNA-dependent RNA polymerases: 1) the multi-subunit polymerases, of which the bacterial RNA polymerase and eukaryotic Pol II are best known and 2) the single subunit RNA polymerases, of which T7 RNA polymerase and the mitchondrial (and chloroplast) RNA polymerases are best known. Although these two broad classes are evolutionarily distinct, the big picture mechanisms are a case of convergent evolution – similarities reflect the constraints of the complex challenge in evolving an enzyme that is initially very sequence-specific, but transitions to an enzyme that can transcribe any sequence.