We studied the sequences transcribed in the rare class of hnRNA and the rare maternal RNA set in blastula embryos and a tissue of adult sea urchins, coelomocytes. About 26 % of labelled single-copy DNA formed hybrids which bound to hydroxyapatite after three cycles of hybridization with nuclear RNA from blastulae and coelomocytes. This corresponds to transcription of about 50 % of the single-copy genome by both cell populations. To compare the rare hnRNA sequences synthesized by blastulae and coelomocytes directly, labelled single-copy DNA was hybridized with blastula nuclear RNA to high RNA C0t, fractionated into sequences complementary and non-complementary to blastula nuclear RNA by chromotography on hydroxyapatite, and then each fraction was rehybridized with nuclear RNA from blastulae and coelomocytes. About 62 % of the labelled DNA complementary to blastula nuclear RNA and about 1·5 % of the labelled DNA non-complementary to blastula nuclear RNA hybridized with nuclear RNA from both cell populations. Thus, coelomocytes and blastula embryos transcribe essentially the same single-copy sequences in the rare hnRN A class. A probe for the rare maternal RNA set was isolated by hybridizing single-copy DNA with total egg RNA to high RNA C0t. 65–67 % of this probe hybridized with whole-cell RNA from eggs, blastulae, plutei and coelomocytes demonstrating that essentially all rare maternal RNAs are present, and presumably transcribed, in blastulae, plutei and coelomocytes.

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