Induction of coiled body-like structures in Xenopus oocytes by U7 snRNA

The coiled bodies, or "sphere organelles," of amphibian oocytes, first identified by their unique morphology, are large structures of up to 10 microm in diameter. There are 40 to 120 coiled bodies per oocyte nucleus. Most of the coiled bodies are in the nucleoplasm but a few are attached to specific chromosomal loci containing the histone gene repeats. Like the coiled bodies of somatic cells, they contain high concentrations of U7 snRNA, a small nuclear RNA required for 3' end formation of histone transcripts, and of a unique protein called coilin/SPH-1. We show here that increasing the nuclear concentration of U7 snRNA, by injection of either in vitro synthesized U7 snRNA or functional U7 snRNA genes, induces the formation of coiled body-like structures in vivo. In contrast, the formation of these structures is not induced when either a promoterless U7 snRNA gene construct is injected or when functional U7 snRNA genes are co-injected with alpha-amanitin. Increasing the concentration of U1 snRNA or histone mRNA does not induce the formation of these structures, indicating that the formation of these coiled body-like structures is specifically induced by the U7 snRNA. These results suggest that U7 snRNA may be a central nucleating factor of coiled bodies and that the appearance of coiled bodies at histone gene loci results from an increased local concentration of U7 snRNA near the nascent histone pre-mRNAs.