Retention of exogenous mRNAs selectively in the germ cells of the sea urchin requires only a 5'-cap and a 3'-UTR.

The abundance of an mRNA in a cell depends on its overall rates of synthesis and decay. RNA stability is an important element in the regulation of gene expression, and is achieved by a variety of processes including specific recruitment of nucleases and RNAi-associated mechanisms. These mechanisms are particularly important in stem cells, which, in many cases, have attenuated transcription. Here we report that exogenous mRNA injected into fertilized eggs of the sea urchin is selectively retained in the small micromeres, which contribute to the germ line in this organism, beginning in blastulae, when compared to adjacent somatic cells. We show that modification of this exogenous RNA using cap analogs and poly-adenosine tail deletions do not affect its selective retention in the small micromeres, but removal of the cap or of the 3'-untranslated region eliminates any selective mRNA retention in the presumptive germ line. Our results illuminate a likely ancient mechanism used by stem cells to prolong the lifespan of RNAs-either through RNA protection or by the absence of basic RNA degradation mechanisms, which are employed by most other cells of an organism.