Closely linked early and late histone H2B genes are differentially expressed after microinjection into sea urchin zygotes.

An early and a late histone H2B gene from the sea urchin Stronglyocentrotus purpuratus were linked in a single plasmid and injected into the eggs of the sea urchin Lytechinus pictus. The levels of transcripts of injected early and late genes and of endogenous early genes were monitored during development by a ribonuclease protection assay. Transcripts of both the injected and endogenous early genes peaked during the blastula stage and decreased severalfold by the mesenchyme blastula stage. Transcripts of the injected late gene became detectable at the blastula stage and increased in amount subsequently, until at least the early gastrula stage, 28 hr after fertilization. Thus, the pattern of expression of the injected early and late H2B genes is similar to that of their endogenous counterparts. These results show that DNA sequences regulating the temporal pattern of early and late H2B gene expression must lie within the cloned DNA segments; i.e., within 600 base pairs of the early H2B gene and 3 kilobases of the late H2B gene.