Automated DNA sequencing on plasmid templates or on PCR products was carried out with dye terminator cycle sequencing kits from Perkin-Elmer on an automated sequencer (model 377, Applied Biosystems), following the manufacturer’s instructions

GST fusions of yeast RNA Pol I subunits were purified as described in (Werneret al., 2010). Yeaststrainsover expressing GST tagged RNA Pol I proteins were grown overnight at 30°C in 10 mL of SC-Ura medium with 2% glucose medium. Cells were pelleted, washed in SC-Ura with galactose. Protein expression was induced by transferring the entire pellet into200 mL of SC-Ura with 2% galatose to give a final OD600of 0.8-1.0. For proteins A190 and A43 that express at very low levels, the overnight culture volume and induction volume were doubled. Cells were cultured at 30°C harvested at 3.0-5.0 OD and washedwith ice cold water. The cell pellet was suspended in 5 mL of ice cold Buffer A (Section 2.1.6.7), 750 μL ofcell suspensions were aliquoted into 1.5 mL microfuge tubes and to this 500 μL glass beads were added. Cells were lysed by bead beating using a vortex mixer (VortexGenei -2 with mix-mate attachment), and the lysate was centrifuged at high speed for 15 min at 4°C. Supernatants were dispensed into a 15 mL conical tube and Triton X-100 was added to a final concentration of 1%. Pre-swollen glutathionebeads were washed in Buffer B(Section 2.1.6.7)from which 200 μL of 1:1 bead suspension was added to approximately 5 mL of A34 and A43 expressing cell lysate and 100 μL of 1:1 bead suspension was added to 5 mL of A190 cell lysate and incubated for 2 h at 4°C on a rotary mixer. Lysates were centrifuged at 5000 xgfor 2 min and the beads were washed with ice cold Buffer C (Section 2.2.6.6) twice. Beads were further washed with ice cold Buffer B followed by ice cold 1X phosphate buffered saline (PBS) twice. Beads were suspended in an equal volume of 1X PBS with protease inhibitor cocktail (Sigma)