Recognition of Protein Synthesis Initiation Signals on Bacteriophage Ribonucleic Acid by Mammalian Ribosomes

Abstract

Previous work has shown that bacteriophage Qβ RNA is translated by cell-free extracts of Krebs II mouse ascites cells. The major product of the reaction is the phage coat protein (M orrison , T. G. & L odish , H. F. (1973) Proc. Natl. Acad. Sci. U. S. A. 70, 315–319; A viv , H., B oime , I., L oyd , B. & L eder , P. (1972) Science 178, 1293–1295). Here we show that ascites cell extracts directed by Qβ RNA also synthesize Qβ replicase and, in addition, a polypeptide which results from initiation of protein synthesis in the interior of the replicase gene. This polypeptide is made in amounts equivalent to that of the replicase. Extracts of ascites cells directed by bacteriophage f2 RNA synthesize the phage coat and replicase, but the major products of the reaction do not co-migrate with authentic f2 protein. These major products are also made in reactions directed by f2 RNA which contain nonsense mutations in any of the three f2 genes. f2 RNA directs incorporation of initiator formyl[35S]methionine into authentic chymotryptic and tryptic peptides of the f2 coat and replicase, but formyl-[35S]methionine is also incorporated into several initiator sequences not found in authentic f2 proteins. These results indicate that ascites ribosomes initiate protein synthesis at both authentic initiation regions and at spurious regions. These incorrect initiations on bacteriophage RNAs by mammalian ribosomes are in marked contrast to the correct initiation by mammalian ribosomes on mammalian viral and cellular RNAs in cell-free systems.