Sindbis virus 26S
RNA is the principal species of virus-specific
RNA found in the infected cell; it is derived from a one third segment of virion 42S
RNA. When translated in cell-free extracts from mouse
ascites cells or rabbit reticulocytes, 26S
RNA directed the synthesis primarily of the 33,000 dalton virus
capsid protein, and the
protein products were in the form of free
peptides rather than
peptidyl-tRNA. In contrast, the
polypeptides synthesized in either extract in response to Sindbis virus 42S
RNA were heterogeneous, ranging in molecular weight from 33,000 to 190,000, and were largely in the form of
peptidyl-tRNA. The number of independent initiation sites on the 26S and 42S RNAs was determined by analyzing a tryptic digest of reaction products labeled with yeast N-formyl-35S-methionyl-tRNAFmet. The 26S
RNA appeared to contain a single initiation site, and this site could also be found in varying amounts in different preparations of 42S
RNA. However, a second initiation site, distinct from that of 26S
RNA, was the major site in 42S virion
RNA. These results suggest that 42S virion
RNA contains two potential sites for initiation of
protein synthesis. Only one of these may be active, however, and it is postulated that the second site functions primarily, if not exclusively, in the subgenomic 26S
RNA species. In this regard, Sindbis virus 42S
RNA may represent a novel form of a eucaryotic
messenger RNA.