TRANSLATION OF CAPPED AND UNCAPPED VESICULAR STOMATITIS-VIRUS AND REOVIRUS MESSENGER-RNAS - SENSITIVITY TO M7GPPPAM AND IONIC CONDITIONS

Abstract

To elucidate the role of the 5''-terminal 7-methylguanosine residue in translation of mammalian mRNA, vesicular stomatitis virus (VS virus) and reovirus mRNA containing and lacking this residue, and also [phage] Q.beta. RNA, were translated in cell-free extracts from reticulocytes and wheat germ under a variety of ionic conditions. Optimal translation of mRNA lacking a 5'' m7G occurred at concentrations of KOAc [potassium acetate] or KCl which were lower than those optimal for normal capped mRNA. This salt dependence was much less marked in the mammalian reticulocyte extract and, at salt concentrations optimal for translation of normal capped mRNA, reticulocyte lysates translated uncapped with mRNA at 30-60% 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. Analogs of the 5'' end, such as m7GpppAm, inhibited translation of both normal and uncapped VS virus RNA in wheat germ extracts to about the same extent, but the efficiency of its action was reduced at low K+ concentrations. Apparently, their is a reduced importance of the 5''m7G residue in ribosome-mRNA recognition at low K+ concentrations, and translation of mRNA in reticulocyte extract is, under any reaction conditions, less dependent on the presence of a 5'' cap than in wheat germ extracts.