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 on the cell-free translation of several leukemia and sarcoma virus RNA was examined. Yeast amber suppressor tRNA (amber tRNA) enhanced the synthesis of the Rauscher murine leukemia virus [R-MuLV] and clone 1 Moloney murine leukemia virus [M-MuLV] Pr200gag-pol polypeptides by 10- to 45-fold, but at the same time depressed the synthesis of R-MuLV Pr65gag and M-MuLV Pr63gag. Under suppressor-minus conditions, M-MuLV Pr70gag was present as a closely spaced doublet. Amber tRNA stimulated the synthesis of the upper M-MuLV Pr70gag polypeptide. Yeast ochre suppressor tRNA appeared 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 M-MuLV Pr200gag-pol and Pr70gag 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 Pr63gag as the mechanism behind the amber tRNA effect. Moloney murine sarcoma virus 124 30S RNA was translated into 4 major polypeptides, Pr67gag, P42, P38 and P23. In the presence of amber tRNA, a new polypeptide, Pr67gag, appeared, whereas Pr63gag synthesis was decreased. Quantitative estimates indicated that for every 1 mol of Pr67gag which appeared, 1 mol of Pr63gag was lost.