Terminal minihelix, a novel RNA motif that directs polymerase III transcripts to the cell cytoplasm - Terminal minihelix and RNA export

Determining the cia-acting elements controlling nuclear export of RNA is cr itical, because they specify which RNA will be selected for transport. We h ave characterized the nuclear export motif of the adenoviral VA1 RNA a smal l cytoplasmic RNA transcribed by RNA polymerase III. Using a large panel of VA1 mutants in both transfected COS cells and injected Xenopus oocytes, we showed that the terminal stem of VA1 is necessary and sufficient for its e xport. Surprisingly, we found that the nucleotide sequence within the termi nal stem is not important. Rather, the salient features of this motif are i ts length and its relative position within the RNA. Such stems thus define a novel and degenerate cytoplasmic localization motif that we termed the mi nihelix, This motif is found in a variety of polymerase III transcripts, an d cross-competition analysis in Xenopus oocytes revealed that export of one such RNA, like hY1 RNA, is specifically competed by VA1 or artificial mini helix, Taken together these results show that the minihelix defines a new c ia-acting export element and that this motif could be exported via a novel and specific nuclear export pathway.