| Abstract: | In an attempt to elucidate the role of the 5'-terminal 7-methylguanosine residue in translation of mammalian mRNAs, vesicular stomatitis virus (VS virus), and reovirus mRNAs containing and lacking this residue, and also Qbeta RNA, were translated in cell-free extracts from reticulocytes and wheat germ under a variety of ionic conditions. Optimal translation of mRNAs lacking a 5' m7G occurred at concentrations of KOAc or KCl which were lower than those optimal for normal "capped" mRNAs. However, this salt dependence was much less marked in the mammalian reticulocyte extract and, at salt concentrations optimal for translation of normal capped mRNAs, reticulocyte lysates translated uncapped with mRNAs at 30 to 60% the normal efficiency. At low K+ concentrations, wheat germ ribosomes bound and translated appreciable amounts of uncapped VS virus mRNA; controls showed that no m7G residue is added to the 5' end of the bound RNA. Analogues of the 5' end, such as m7GpppAm, inhibited translation of both normal and uncapped VS virus RNAs in wheat germ extracts to about the same extent, but the efficiency of its action was reduced at low K+ concentrations. We conclude that there is a reduced importance of the 5' m7G residue in ribosome-mRNA recognition at low K+ concentrations, and that translation of mRNAs in reticulocyte extract is, under any reaction conditions, less dependent on the presence of a 5' "cap" than in wheat germ extracts. |
