SEA-URCHIN EARLY HISTONE H2A MODULATOR BINDING FACTOR-I IS A POSITIVETRANSCRIPTION FACTOR ALSO FOR THE EARLY HISTONE H3 GENE

To shed some light on the mechanisms involved in the coordinate regula tion of the early histone gene set during sea urchin development, we t ested the hypothesis that the upstream sequence element USE1, previous ly identified in the early H2A modulator, could also participate in th e transcription of the early histone H3 gene. We found by DNase I prot ection analysis and by competition in electrophoretic mobility-shift e xperiments that two sequence elements of the H3 promoter closely resem bled the USE1-H2A sequence in their binding activity for nuclear facto rs from 64-cell stage embryos. These modulator binding factor 1 (MBF-1 )-related factors seem to recognize the ACAGA motif that is conserved between the USE1-like sequences of both H2A and H3 promoters. In fact, excess oligonucleotide containing a mutated USE1-H2A element in which the ACAGA sequence was mutated to AGTCA failed to compete with the US E1 sites of both H2A and H3 genes for interaction with MBF-1. Finally, in vivo transcriptional analysis in both Xenopus and sea urchin showe d that an excess of USE1-H2A element efficiently competed for the acti vity of the H3 promoter. From these results we conclude that MBF-1 is a transcription factor conserved between sea urchin and frog and that MBF-1 or related transcription factors are involved in the coordinate expression of both H2A and H3 early histone genes.