Delayed recruitment of maternal histone H3 mRNA in sea urchin embryos

Abstract

During the early stages of embryogenesis, the sea urchin embryo uses maternally synthesized mRNA stored in the egg as inactive messenger ribonucleoproteins (mRNPs)¹. Release of this mRNA for translation allows the embryo to develop even in the absence of new mRNA synthesis. Comparison using two-dimensional gel electrophoresis shows that the same spectrum of prevalent proteins are synthesized by eggs and zygotes 30–60 min after fertilization². Thus, unlike the case of the clam Spisula in which there are several prominent changes in translation associated with fertilization³, sea urchins have been thought to show little or no regulation of translation of specific mRNA sequences. Histone mRNAs are major components of the mRNA pool, comprising as much as 4–8% of the total mRNA of eggs⁴, yet their products, histones, would not normally be detectable on two-dimensional gels. We report here that mRNA complementary to a histone H3 cloned probe remains in the inactive mRNA pool for 90 min after fertilization before it begins to be translated. This is long after the rapid increase in overall protein synthesis, using stored mRNAs, has begun. As we can demonstrate no significant synthesis of histone H3 mRNA during this period, stored histone H3 mRNA must be subject to sequence-specific translational regulation.