The nature of transcripts from repetitive DNA sequences in the sea urchin,Strongylocentrotus purpuratus, is investigated. Hybridization experiments utilizingindividual cloned repeat sequences, as well as fractions of total repetitive DNA,indicate that the expression of repeat sequences in RNA is specifically regulatedin development. A different set of repeat families is highly represented in eachof three RNA populations examined, the nuclear RNAs of gastrula stage embryosand adult intestine tissue, and the total RNA of eggs. Essentially all the genomicrepeat families are represented in each RNA, but the prevalence of transcriptsrepresenting different repeat families can vary by more than two orders of magnitudein a given RNA. Both complementary strands of most repeat families are representedat similar levels, raising the possibility that RNA-RNA repeat duplex formationoccurs in the cell. Two cloned repeat sequences examined were both found primarilyon large transcripts in the nuclear RNA, and many of the nuclear repeat transcriptsare believed to occur on long interspersed RNA molecules.
Several lines of evidence indicate that most repeat sequences in the eggRNA are contained on transcripts with the properties of maternal messenger RNA.A large fraction of the repeat-containing transcripts are polyadenylated. Most ofthe repeats are found on long transcripts, while in the genome, these repeats areshort and interspersed with single-copy sequences. The repeat-containing RNAsare isolated and directly shown to consist of short repeats linked to longer single-copysequences. These interspersed egg RNAs are shown to include nearly all ofthe diverse single-copy sequences of total egg RNA, most of which are believedto be message sequences. Several implications of these findings are discussed.Particularly interesting is the conclusion that the single-copy maternal messagesequences must be associated primarily with a restricted group of the diversegenomic repeat families. The message sequences thus fall into several hundredsets, each containing transcripts from a different repeat family.
