Metabolism of low molecular weight ribonucleic acids in early sea urchin embryos.

There are three major low molecular weight RNAs (150-300 nucleotides) larger than 5S rRNA present in sea urchin embryos. Two of these are localized in the nucleus and one is localized in the cytoplasm. The nuclear species contain "capped" 5' termini, with a cap I structure. These RNAs are synthesized starting in late cleavage and continuing through pluteus. Relative to 5S RNA there is a 10-fold change in the rate of synthesis of these RNAs, due primarily to a decrease in their rate of transcription after blastula. The RNAs are metabolically stable and the nuclear RNA genes are reiterated 50--100 times in the genome. Significant amounts of these RNAs are present in sea urchin eggs, enough to supply the embryo during early cleavage, prior to initiation of their synthesis.