Nucleic acid polymerase structure has been studied by both X-ray and electron crystallography. To date, only the smaller, single subunit polymerases have been subjected to X-ray analysis, including the bacteriophage T7 RNA polymerase, which is the only RNA polymerase whose structure is known to atomic resolution. Lower resolution structures of several multisubunit polymerases have been determined by electron crystallography, including a mutant form of yeast RNA polymerase II which lacks subunits Rpb4 and Rpb7 (denoted A4/7 polymerase). All polymerase structures obtained by both X-ray and electron crystallography show a large cleft appropriate in size for binding duplex DNA, and further appear to contain a mobile arm allowing open and closed conformations of the cleft, presumably permitting entry and retention of DNA. Subunits Rpb4 and Rpb7 of RNA polymerase II form a dissociable subcomplex that has been implicated in the stress response and in the initiation of transcription. Human homologs of Rpb4 and Rpb7 have been identified.