The role of cap methylation in the translational activation of stored maternal histone mRNA in sea urchin embryos.

Cap methylation was examined in the early sea urchin embryo. Nucleotide analyses of 3H-methyl methionine-labeled RNA in two-cell embryos and in unfertilized eggs show that fertilization activates the cap methylation of about 10(7) RNA molecules. Greater than 37% of methyl-labeled RNAs following fertilization hybridize with so-called early histone genes H1, H4, and H2B, which encode a subpopulation of the maternal mRNA molecules. Activation of RNA cap methylation is inhibited by aphidicolin, but not by actinomycin D, suggesting that this process is temporally coordinated with DNA replication, but independent of RNA transcription. These results indicate that the translational activation of maternal early histone mRNA during fertilization is a consequence of cap methylation of mRNAs incompletely formed during oogenesis.