Early cleavage-stage embryos of Xenopus laevis were treated with the cleavage inhibitors colchicine and cytochalasin B, and the synthesis of rRNA in the cleavage-arrested embryos was analyzed in order to examine the temporal control of rDNA expression. Cytochalasin B arrested early cleavage, but nuclear division and DNA replication continued to some degree. Colchicine inhibited not only cleavage but also nuclear division; rDNA content in the colchicine-arrested embryos at gastrula stage was about 4% of that in control. However, rRNA synthesis in these arrested embryos was activated as in the control embryos and the rate of rRNA synthesis in the arrested embryos seemed to be nearly equal to that of control embryos. These results show the independence of rRNA gene activation from the number of cleavages, from DNA replication, and from the ratio of nucleus to cytoplasm.
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