FIGURE 3.

rRNA synthesis is reduced in strains that lack HPR1 or THO2. A, cells from WT (DAS886), hpr1Δ (DAS888), or tho2Δ (DAS887) were pulse-labeled with [³H]uridine for 5 min. Total RNA was purified and run on a formaldehyde-agarose gel. B, rRNA mature species and tRNA were excised and counted in a scintillation counter. Cells from WT (DAS396), hpr1Δ (DAS632), or tho2Δ (DAS677) were pulse-labeled with [methyl-³H]methionine for 6 min (P for pulse), and half of the cells were harvested. After an additional 2 min (8 min total pulse), the remaining culture was treated with excess cold methionine and harvested after an additional 5 min of growth (C for chase). RNA was purified, electrophoresed, and transferred to a membrane. After detection by autoradiography, 25S and 18S rRNA were cut from the membrane and quantified with a scintillation counter. Positions of mature and precursor species are indicated. C, growth rate and total RNA were measured in WT and THO mutants and expressed relative to WT. The predicted rRNA synthesis rate was calculated by multiplication of growth rate by total RNA abundance relative to WT. The observed rRNA synthesis rate is measured by quantification of 25S and 18S in B.