Messenger RNA synthesis during early ascidian development.

RNA synthesis during early embryogenesis of the ascidian Ciona intestinalis was studied. Embryonic polyribosomes labeled with uridine from 5 to 7 hr after fertilization were isolated and the labeled RNA species were characterized by oligo(dT)-cellulose chromatography and sucrose gradient sedimentation analysis. Since at least 50% of the labeled RNA was polyadenylated and all of it sedimented heterogeneously, it was concluded that mRNA was synthesized during the labeling period. Further, the synthesis of heterogeneously sedimenting, polyadenylated RNA at various stages of development from midcleavage to metamorphosis indicated that gene activity and perhaps mRNA synthesis occurred at earlier and later stages of development as well. Autoradiographic studies showed that the embryonic genome was the site of this activity, since uridine incorporation was localized in embryonic cells and not in accessory cells. Finally, under the labeling conditions employed (2-hr pulses), rRNA synthesis was not detected until larvae hatched.