Functional Architecture of RNA polymerase I

Synthesis of ribosomal RNA by RNA polymerase (Pol) I is the first
step in ribosome biogenesis and a regulatory switch in eukaryotic
cell growth. In this thesis a reproducible large-scale purification
protocol for Pol I from S. cerevisiae could be developed. Crystals
were obtained, diffraction to < 4 Å could be recorded, however,
the enormously complex non-crystallographic symmetry impeded
structure solution. Switching to cryo-electron microscopy, the
structure of the complete 14-subunit enzyme could be solved to 12 Å
resolution, a homology model for the core enzyme could be
generated, and the crystal structure of the subcomplex A14/43 could
be solved. In the resulting hybrid structure of Pol I, A14/43, the
clamp, and the dock domain contribute to a unique surface
interacting with promoter-specific initiation factors. The Pol
I-specific subunits A49 and A34.5 form a heterodimer near the
enzyme funnel that acts as a built-in elongation factor, and is
related to the Pol II-associated factor TFIIF. In contrast to Pol
II, Pol I has a strong intrinsic 3’-RNA cleavage activity, which
requires the C-terminal domain of subunit A12.2, and apparently
enables rRNA proofreading and 3’-end trimming.