The sea urchin histone H2A enhancer-binding protein MBF-1 is needed for maximal expression also for the H3 gene, while is buffered by the sns5 insulator on its interaction with the downstream H1 promoter.

Enhancers are DNA elements which increase the transcription of associated gene in a position and distance independent manner relative to the transcription initiation site. Molecular mechanisms must operate to direct enhancers to specific promoters in complex genetic loci. The sea urchin a-histone genes are organized in several hundred tandem repeated units, each containing one copy of the five histone genes in the order 5’-H4-H2B-H3-H2A-H1-3’. Transcription is limited to the early cleavage and reaches its maximum at morula stage. After hatching these genes are repressed and maintained in the silenced state for whole life cycle of the animal. In Paracentrotus lividus, the MBF-1 activator binds to a 30 bp sequence of the H2A modulator, the only cis-acting element of the histone unit for which an enhancer activity has unambiguously demonstrated. Here we assessed the specificity of the MBF-1 factor and the molecular mechanisms that eventually limit its interaction with the adjacent promoters. We find by Chromatin ImmunePrecipitation and in vivo functional studies that the MBF-1 activator binds both the H3 and H2A regulatory sequences, suggesting that this activator is needed for the maximal expression of both genes. In addition, we find that the enhancer blocking activity of the sns5 insulator buffers the downstream H1 promoter from the H2A enhancer bound MBF-1 factor. These results suggest that the block of the H2A enhancer by the sns5 insulator might be the mechanism that regulate the ratio of 2 to 1 of core to linker histones during early development of the sea urchin embryo.