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Abstract:
Transcription of ribosomal RNA by RNA polymerase (Pol) I initiates ribosome biogenesis and regulates eukaryotic cell
growth. The crystal structure of Pol I fromthe yeast Saccharomyces cerevisiae at 2.8A˚ 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.
