ABSTRACT
Three DNA-dependent RNA polymerases are responsible for the synthesis of eukaryotic mRNAs. The most important is RNA polymerase II (RNA Pol II), which is involved in the transcription of all protein genes and most small nuclear (sn) RNA genes. RNA Pol II from a typical eukaryotic cell contains up to 15 polypeptide subunits and has a molecular mass up to 600 kDa. Despite the complexity and huge size of eukaryotic RNA polymerases, important insights into various aspects of the transcription mechanisms have been gained from comparison of the three-dimensional structure of a bacterial and a yeast RNA polymerase bound to nucleic acids.
