The programmed switch in lysine-rich histone synthesis at gastrulation

The pattern of histone synthesis changes during development in sea urchins. One change involves two different lysine-rich histones. In Lytechinus, the late appearing histone, H1_g, begins to be synthesized at gastrulation. Preformed ("maternal") mRNA of unfertilized eggs contains sequences that direct the synthesis of a part of the histone made during the period of cleavage. During that interval, a different lysine-rich histone, H1_m, is produced. Maternal mRNA is not, however, the sole source of templates for histone synthesis. Transcription of embryonic genomes provides the major share, and cleavage stage polyribosomes translate both maternal and embryonic messages. A cell-free system derived from wheat germ was used to test RNAs from unfertilized eggs and from post-gastrula embryos for their capacity to direct the synthesis of histones, including the H1 types, in vitro. Maternal mRNA includes templates for the synthesis of H1_m, but it appears to lack those for H1_g, since the cell-free system yields only H1_m when challenged with egg RNA. Since the cell-free system is capable of translating H1_g mRNA when presented with it, the synthesis of H1_g is probably a very early developmental event controlled at the level of transcription.