Transcrption of ribosomal RNA by RNA polymerase(Pol) I initiates ribosome biogenesis and regulates eukaryotic cell growth.The crystal structure of Pol I from the yeast Saccharomyces cerevisiae at 2.8A resolution reveals all 14 subunits of the590-kilodalton enzyme, and shows differences to Pol II.A expander element occupies the DNA template site and stabilizes an expanded active centre cleft with an unwound bridge helix.A connector element invades the cleft of an adjacent polymerase and stabilizes an inactive polymerase dimer.The connector and expander must detach during Pol I activation to enable transcription initiation and cleft contraction by convergent movement of the polymerase core and shelf modules.Conversion between an inactive expanded and an active contracted polymerase stste may generally undelie transcription.Regulatory factors can modulate the core-shelf interface that includes a composite active site for RNA chain initiation,elongation,proofreading and termination.