An absolute requirement for the 5′ cap structure for mRNA translation in sea urchin eggs

Abstract

Translation of a variety of RNA was studied in a cell-free translation system derived from sea urchin eggs. While RNA such as globin or tobacco mosaic virus are efficiently translated, viral RNA which do not contain the 5'' cap structure, such as cow pea mosaic virus (CPMV) and poliovirus, are not translated. Mixing experiments with reticulocyte lysates indicated that the lack of translation of uncapped viral RNA is not due to the presence of a potent inhibitor or the absence of an activating agent. RNA competition experiments between capped and uncapped RNA indicated that uncapped RNA do not interact with the sea urchin egg initiation machinery. Proteolytic removal of the 5'' viral protein did not allow the translation of CPMV RNA. Chemical decapping of vesicular stomatitis virus mRNA completely inhibited the translation of this mRNA in the sea urchin cell-free system. The sea urchin egg lacks the initiation pathway used to initiate uncapped mRNA in mammalian cells and thus, has an absolute requirement for the 5'' cap structure for initiation. The implications of these findings for the control of protein synthesis after fertilization of the sea urchin egg are discussed.