Transcription 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 2.8 Å resolution reveals all 14 subunits of the 590-kilodalton enzyme, and shows differences to Pol II. An `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 state may generally underlie transcription. Regulatory factors can modulate the core-shelf interface that includes a `composite' active site for RNA chain initiation, elongation, proofreading and termination.