Importance of initiation factor preparations in the translation of reovirus and globin mRNAs lacking a 5'-terminal 7-methylguanosine.

Reovirus and globin mRNAs which lack a 5'-terminal 7-methylguanosine are translated in a fractionated cell-free protein-synthesizing system. Efficient translation occurs only at optimal concentrations of reticulocyte initiation factor preparations and does not occur at optimal concentrations of ascites initiation factor preparations or at suboptimal concentrations of reticulocyte initiation factor preparations. The translation of "uncapped" mRNA in vitro, therefore, appears to be related to the efficiency of initiation of protein synthesis. At optimal concentrations of reticulocyte initiation factors, mRNA containing a 5'-terminal 7-methylguanosine is preferentially translated in the presence of mRNA which lacks a "cap." These results indicate that the 5'-terminal 7-methylguanosine on mRNA has a facilitatory rather than obligatory role in translation.