A developmental switch in sea urchin U1 RNA.

The sequence of U1 RNA has been determined in the eggs and embryos of two sea urchins, Lytechinus variegatus and Strongylocentrotus purpuratus. In both species the sequence of the U1 RNA changes as the embryos progress through development. The sequence of the major U1 RNA in the eggs of the two species differs in two nucleotides, while the sequence of the U1 RNA present in the late embryos and somatic tissue is identical in the two species. The U1 RNA in eggs and early embryos is primarily derived from the tandemly repeated gene set, which is not expressed in somatic tissues.