Suppression of Murine Retrovirus Polypeptide Termination: Effect of Amber Suppressor tRNA on the Cell-Free Translation of Rauscher Murine Leukemia Virus, Moloney Murine Leukemia Virus, and Moloney Murine Sarcoma Virus 124 RNA

Abstract

The effect of suppressor tRNA's on the cell-free translation of several leukemia and sarcoma virus RNAs was examined. Yeast amber suppressor tRNA (amber tRNA) enhanced the synthesis of the Rauscher murine leukemia virus and clone 1 Moloney murine leukemia virus Pr200 gag-pol polypeptides by 10- to 45-fold, but at the same time depressed the synthesis of Rauscher murine leukemia virus Pr65 gag and Moloney murine leukemia virus Pr63 gag . Under suppressor-minus conditions, Moloney murine leukemia virus Pr70 gag was present as a closely spaced doublet. Amber tRNA stimulated the synthesis of the “upper” Moloney murine leukemia virus Pr70 gag polypeptide. Yeast ochre suppressor tRNA appeared to be ineffective. Quantitative analyses of the kinetics of viral precursor polypeptide accumulation in the presence of amber tRNA showed that during linear protein synthesis, the increase in accumulated Moloney murine leukemia virus Pr200 gag-pol coincided closely with the molar loss of Pr63 gag . Enhancement of Pr200 gag-pol and Pr70 gag by amber tRNA persisted in the presence of pactamycin, a drug which blocks the initiation of protein synthesis, thus arguing for the addition of amino acids to the C terminus of Pr63 gag as the mechanism behind the amber tRNA effect. Moloney murine sarcoma virus 124 30S RNA was translated into four major polypeptides, Pr63 gag , P42, P38, and P23. In the presence of amber tRNA, a new polypeptide, Pr67 gag , appeared, whereas Pr63 gag synthesis was decreased. Quantitative estimates indicated that for every 1 mol of Pr67 gag which appeared, 1 mol of Pr63 gag was lost.