Histone gene switch in the sea urchin embryo. Identification of late embryonic histone messenger ribonucleic acids and the control of their synthesis

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Abstract
During embryogenesis in the sea urchin Strongylocentrous purpuratus, there is a shift from one histone mRNA population to another. The early and late embryonic histone mRNA, previously shown to differ considerably in sequence from each other by hybrid melting studies, differ also in electrophoretic mobility on polyacrylamide gels as the positions of the early and late mRNA are completely noncoincident. The various species of both early and late samples are identified as particular histone mRNA by hybridization to cloned histone DNA containing part of the early-type repeat unit or to restriction enzyme fragments derived from these units. Four bands in the early mRNA sample are identified as H1, H3, H2A + H2B and H4 mRNA, while at least 10 bands can be seen in the late mRNA preparation with unambiguous identification of H1, H2B and H4 mRNA. A cluster of late species contained both H3 and H2A mRNA. When a polysomal RNA preparation from the 26 h embryo is hybridized to the histone DNA, eluted and then translated in vitro in a wheat germ system, the histone products migrate in the position of late histones when subjected to electrophoresis on Triton X-urea gels. DNA which contains genes for H2A + H3 or H2A alone was used to demonstrate the specificity of the early-type DNA probes for these 2 late histones. By hybridization of newly synthesized RNA and translation of the total polysomal RNA present in the late embryo, it is shown that mRNA for all 5 histone classes may cross-react with the cloned early-type DNA. The hybrids formed are much less stable than those formed with the early histone mRNA. In vitro translation of total cytoplasmic RNA from various embryonic stages indicates that transition between the 2 classes occurs during most of the blastula period.