Synthesis of histone mRNA sequences in isolated nuclei of cleavage stage sea urchin embryos

A qualitative assay for detection of histone mRNA sequences in nuclear RNA was developed using actinomycin D-CsCl gradients to separate histone DNA from bulk DNA by differences in buoyant density. A significant amount of RNA synthesized in vitro in isolated nuclei from early blastula stage sea urchin embryos hybridized coincident with the histone DNA satellite, and this hybridization was competed out by unlabeled “9S” polysomal RNA purified from embryos at the same stage of development. The biogenesis of these histone mRNA sequences appeared similar as observed during in vivo and in vitro synthesis. Nuclear RNA from embryos pulse labeled in vivo was found to lack histone sequences, suggesting a rapid exit time for these sequences from the nucleus. Attempts to study the exit of histone sequences from isolated nuclei labeled in vitro also suggested a rapid exit time for histone sequences. The histone sequences were synthesized to a much lesser extent in isolated nuclei from late blastula stage embryos, as anticipated from the much reduced amount of histone mRNA labeled on polysomes at this stage.