Gene expression during early embryogenesis of sea urchin: The histone and homeobox genes

Transcriptional regulators are thought to play a key role in cell fate determination and territorial specification in sea urchin. Our goals are to clone transcription factors for studying embryonic development. One approach has been to use promoter binding and gene transfer technology to investigate the mechanisms of transcriptional activation and repression of the early H2A histone gene. By this analysis we identified a transcriptional activator, the MBF-1, that binds to the modulator element of the H2A gene and enhances the activity of the H2A promoter. However, the enhances activity of the modulator and its interaction with MBF-1 also occurs at the gastrula stage when the early histone genes are shut off. Therefore, the silencing of the early H2A histone gene at late stages of development requires the inactivation of the modulator function. To search for antimodulator sequence elements, we took advantage of our previous work showing the presence of phased nucleosomes specifically positioned on the 3'-spacer and in the modulator of the repressed H2A gene. Evidence is described indicating that a 3'-spacer DNA fragment cloned between the modulator and the basal promoter behaves as an antimodulator element. However, this element does not confer temporal capability to the modulator in its function, suggesting that other elements have to be involved in the regulation of the early H2A expression. The second approach relied on the cloning of genes controlling development using probes of regulators known to be involved in regional specification, such as the homeobox genes, with the aim to understand their possible role through the study of their temporal and territorial expression and through the analysis of the mechanism of regulation of their expression. We present evidence that PlHbox12, a homeodomain encoding gene, is transiently expressed during the early/mid cleavage stages. The abundance of the transcripts reach their maximum in embryos at the 64/128-cell stage concomitantly with the segregation of the specified embryonic territories. Expression of PlHbox12 is drastically reduced in the absence of cell contacts and/or Ca ions, suggesting that this gene is transcriptionally activated by signal transduction mechanisms. Whole mount in situ hybridization showed that PlHbox 12 transcripts are asymmetrically distributed along the A-V axis, being spatially localized in the blastomeres of the ectodermal lineage.We suggest that PlHbox12 might be involved in the initial events that lead to the specification of the ectodermal territories.