Activation of the L1 late H2B histone gene in blastula-stage sea urchin embryos by Antennapedia-class homeoprotein.

The L1 late H2B histone gene of the sea urchin Strongylocentrotus purpuratus is transcriptionally activated in late blastula stage embryos by a mechanism that depends on an enhancer element located 3' of the gene (Zhao et al., 1990). A protein factor, designated H2B abp 1, binds this element at a site that resembles the consensus recognition sequence of Antennapedia-class homeodomain proteins. We demonstrate here that Antennapedia (Antp) and Hbox4 proteins, members of the Antennapedia class of homeoproteins from Drosophila and sea urchin respectively, bind the L1 H2B abp 1 site, and that the Drosophila Antp protein acts through this site to trans-activate the L1 H2B gene, in vivo. In addition, RNA gel blot analysis demonstrated that Hbox4 transcripts accumulate in developing embryos with a time course that closely resembles that of H2B adp 1 DNA binding activity and the activity and the transcription rate of the L1 late H2B gene. Finally, we show that antibody prepared against the sea urchin Hbox4 protein, a member of the Abd-B subclass of the Antennapedia class, specifically inhibits binding of the H2B abp 1 factor to the L1 H2B enhancer, suggesting that H2B abp 1 is encoded by Hbox4 or a closely related gene.