Accumulation of histone repeat transcripts in the sea urchin egg pronucleus.

RNA transcripts complementary to a genomic histone repeat are found in high concentration in sea urchin egg pronuclei. In situ hybridizations with the recombinant plasmid pCO2 indicate that the nuclear concentration is at least 25 to 50 fold higher than that in the cytoplasm. If nuclear transcripts are predominantly histone mRNAs, they comprise about 12% of the histone mRNA in eggs, or about 0.36 pg. After fertilization these molecules persist through pronuclear fusion but disappear from nuclei by mid 2-cell stage. A similar high nuclear concentration is not observed for polyadenylated mRNAs. The high steady-state concentration of nuclear histone repeat transcripts suggests that they have an unusually long lifetime in pronuclei of unfertilized sea urchin eggs.