Sea urchin embryo mRNA sequences expressed in the nuclear RNA of adult tissues

The representation of message sequences in nuclear RNA was studied in sea urchin tissues which utilize these messages in their polysomes and in tissues which do not. A ³H-labeled single-copy DNA tracer highly enriched for sequences complementary to blastula embryo mRNA was prepared. This tracer (mDNA) reacted to 78% with excess blastula mRNA, compared with 2.1% for the starting single-copy DNA. As expected from previous data, the mDNA reacted to only 12% with cytoplasmic RNA of adult intestine, since most embryo mRNA sequences are not detectable in this tissue. The mDNA reacted to the same extent, however, with nuclear RNA from either adult intestine, adult coelomocytes or gastrula embryos as with polysomal mRNA from blastula embryos. Thus virtually all of the blastula mRNA sequences are present in adult tissue nuclear RNAs, although most of these mRNA sequences are absent from cytoplasmic or polysomal RNA in the adult cells. The blastula mRNA sequences are present in heterogeneous nuclear RNAs at the same concentrations as are nuclear transcripts of most single-copy DNA sequences. Calculations based on the steady state concentration of structural gene transcripts in the embryo nuclei suggest that a majority of these molecules do not serve as message precursors.