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Figure S1. Protein synthesis elongation rates. Yeast strains were transformed with pDN317 (Ng, D.T.W., J.D. Brown, and P. Walter. 1996. J. Cell Biol. 134:269–278) that encodes DPAPB-HA under control of the glyceraldehyde 3-phosphate dehydrogenase promoter. Cells were pulse labeled at 30°C as described in Materials and methods. At frequent time points (30 s to 5 min), cells (4 A₆₀₀) were removed, and the labeling reaction was terminated by adding an equal volume of chilled 20 mM NaN₃ plus unlabeled cysteine and methionine to 0.6mg/ml followed by freezing in liquid nitrogen. Total incorporation of Tran-³⁵S-label into protein was determined by TCA precipitation. Radiolabeling of full-length DPAPB-HA was determined by immunoprecipitation followed by SDS-PAGE and detection with a molecular imager. (A) Time course of incorporation of Tran-³⁵S-label into total protein (circles) and into DPAPB-HA (squares) by the SRβ-ΔTM yeast strain. Extrapolation of the linear part of the incorporation curves to the abscissa yielded lag times for incorporation of Tran-³⁵S-label into total protein and into DPAPB-HA. The difference between these two lag times equals half the time required to synthesize DPAPB-HA (Horwitz, M.S., M.D. Scharff, and J.V. Maizel Jr. 1969. Virology. 39:682–694). (B) The protein synthesis rate (residues-s⁻¹) was calculated by dividing the residues in DPAPB-HA by the synthesis time. With the exception of the wild type (wt) and sbh1ΔSRβ-ΔTM mutant, plotted values represent single determinations from experiments wherein three or more yeast strains were analyzed in parallel. Replicate values for the wild type and sbh1ΔSRβ-ΔTM mutant were within 10% of the plotted value. Elongation rates for DPAPB and CPY (not depicted) were similar in the SRα-null strain. Error bars represent SD.