Release factors differing in specificity for terminator codons.

In a previous paper, initiator and terminator trinucleotides were shown to stimulate sequentially the binding of f-Met-tRNA to ribosomes and the release of free f-methionine from the ribosomal intermediate.1 The formation of f-methionine was dependent upon a terminator codon, such as UGA, UAA, or UAG, and the release factor R discovered by Capecchi.2 The separation of R into two components is described in this report. R1 corresponds to the codons UAA and UAG; R2, to UAA and UGA. Methods. -R assay: The termination assay is described elsewhere.1 Each reaction contained the following components in a final volume of 50 Al: 0.05 M Tris-acetate, pH 7.2; 0.03 M magnesium acetate; 0.05 M potassium acetate; 4-6 AMmoles of the (f ['H]Met-tRNAf .AUG.-.Ribosome) complex (4.0-6.7 A/Amoles of TCA-precipitable f ['H]Met-tRNAf, 0.02 A2W0 unit was present/reaction); 0.96 A260 unitof E. coli B ribosomes (washed with 0.5 M ammonium chloride); 0.17 mumole of AUG; and, where indicated, 7-7.7 mInmoles of terminator trinucleotide, and an R preparation. Reactions were incubated for 16 min at 30° unless otherwise stated; hence, the rate of f [3H]-methionine formation was determined in all experiments. The preparation of tRNAfMet (E. coli B), fractionated by benzoylated-DEAE-cellulose column chromatography,' was the gift of Dr. Michael Wilcox. The tRNAfmet was acylated with ['H]-methyl-methionine (3.1 c/mmole, Schwarz BioResearch Corp.) and then converted to f['H]-Met-tRNAfMet.l Ribosomes were obtained from E. coli B as described by Lucas-Lenard and Lipmann,4 except that seven, rather than five, 0.5 M ammonium chloride washes were employed. R fractionation: Escherichia coli B supernatant and ribosome fractions, treated with DNase but not "preincubated," were prepared as described previously,5 except that cells were lysed with a French pressure cell at 18,000 psi, 0.005 M DTT replaced ,B-mercaptoethanol, and extracts were centrifuged for 5 hr at 137,000 X g. Release factors (prep. A, Table 1) were obtained as follows: R was precipitated by the addition of 116 gm of ammonium sulfate to 359 ml of the S-137 fraction; the pH was maintained at 7.8 by the addition of 0.5 N ammonium hydroxide. All steps were performed at 4°. The precipitate was collected by centrifugation at 30,000 X g for 15 min, dissolved in 139 ml of buffer A (0.05M Tris-chloride, pH 8.0; 0.15 M potassium chloride; 0.001 M EDTA, adjusted to pH 7 with NaOH; and 0.003 M DTT) and dialyzed against 4 liters of buffer A for 4 hr (0-55% (NH4)2S04 fraction of Table 1). DEAE-Sephadex column: The 0-55% (NH4)2S04 fraction (138 ml, 2200 mg protein) was diluted to 275 ml with buffer A and applied to a DEAE-Sephadex A-50 column (112 X 2.8 cm) equilibrated with buffer A at a flow rate of 0.5 ml/min. Protein was eluted with 3200 ml of buffer A that contained a linear KCl gradient (0.15-0.70 M). R fractions (prep. B) illustrated in Figures 1 and 2 were obtained by a slightly different procedure.1 The 0-35% (NH4)2SO4 fraction (450 mg protein)-dissolved in 0.01 M Trischloride, pH 8.0; 0.001 M EDTA; 0.001 M DTT; and 0.2 M KCl; (buffer B)-was applied to a 36 X 2.8-cm DEAE-Sephadex A-50 column. Protein was eluted with 1300 ml of buffer B that contained a linear KCl gradient (0.2-0.6 M) at a flow rate of 0.5 ml/min. Each fraction contained 11 ml. Sephadex G-100 column (Fig. 2): Portions of R1 or R2 (prep. B, DEAE-Sephadex fractions) were applied to a column (85 X 1.8 cm) packed with Sephadex G-100 (bead