Message-specific sequestration of maternal histone mRNA in the sea urchin egg

Nucleate and anucleate fragments of sea urchin eggs were prepared by centrifugation on sucrose step gradients. The amount of total RNA, poly(A)+ RNA, histone mRNA, actin mRNA, alpha-tubulin mRNA, and mitochondrial rRNA was determined for each fragment. Total RNA, poly(A)+ RNA, actin mRNA, and alpha-tubulin mRNA all distributed in the same ratio as the volume of the fragments. In contrast, the mitochondrial rRNA was found preferentially distributed in the anucleate fragments, coinciding with the distribution of the mitochondria. Histone mRNAs did not follow the fragment volume ratios, but rather were always found associated with the fragment containing the nucleus. To distinguish between nuclear association and possible artifacts associated with centrifugation, eggs were manually cut into nucleate and anucleate fragments and the amount of histone mRNA was determined for each set. Again only the fragments containing the nucleus had detectable amounts of histone mRNA. Although histone mRNAs were always associated with the nucleate fragment, very little histone mRNA was found associated with isolated egg nuclei prepared under gentle isotonic isolation conditions. Furthermore, embryos that have had first nuclear breakdown blocked with 6-dimethylaminopurine still initiated the recruitment of histone mRNAs into polysomes at the same time as control embryos, thus indicating that nuclear breakdown is not necessary for normal histone message utilization. These results demonstrate a message-specific sequestration of maternal histone mRNA which is physically different from that of other maternal mRNAs and which may govern the timing of maternal histone synthesis in sea urchin embryos.