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Normal development requires that individual genes be expressed in their correct temporal patterns, but the mechanisms regulating this process during early embryogenesis are poorly understood. We have studied the early and late sea urchin histone genes during embryogenesis to address the molecular mechanisms controlling temporal gene expression. By measuring the changes in expression of cloned H1-beta DNA constructs after microinjection into fertilized one-cell zygotes, we demonstrated that a highly conserved 30-base-pair segment of DNA between positions -288 and -317 (USE IV) is responsible for the transcriptional activation of this late histone gene at the late blastula stage. In this report, we demonstrate that an oligonucleotide corresponding to USE IV acts as an embryonic enhancer element capable of activating the simian virus 40 early promoter in a stage-specific manner. Using an in vivo competition assay and in vitro DNase I footprinting and mobility shift assays, we also identified a protein(s) that interacts with this enhancer. Results of the competition assay suggested that this factor acts to stimulate transcription of the H1-beta gene. The factor was found to be stored in mature eggs as well as in all embryonic stages examined. The mobility of the factor found in eggs, however, differed from that of the embryonic form, which suggested that posttranslational modification occurs after fertilization.

We have cloned and characterized the gene encoding the late histone H1-beta subtype from the sea urchin Strongylocentrotus purpuratus. The gene contains all of the upstream sequence homologies previously seen in late H1-gamma genes. The expression of H1-beta mRNA is coordinated with that of H1-gamma mRNA, and like H1-gamma it is expressed in all adult somatic tissues tested.

We cloned and characterized the gene encoding H1-gamma, a late histone subtype of the sea urchin species Strongylocentrotus purpuratus. The predicted primary sequence of H1-gamma is 216 amino acids in length and has a net charge of +70, which is high for a somatic H1 histone. The H1-gamma gene appears to be a unique sequence gene that is not tightly linked to the core histone genes. The 770-base-pair transcribed region of the H1-gamma gene is bordered on the 5' side by two previously described H1-specific sequence elements and on the 3' side by a hairpin loop structure and CAGA box sequences. We detected 3,900 stored maternal H1-gamma mRNA transcripts per egg. The number of H1-gamma transcripts per embryo rises by 9.5 h postfertilization, but the maximum rate of accumulation (4,300 molecules per min per embryo) occurs in the late-blastula-stage embryo between 14 and 21 h after fertilization. The number of H1-gamma mRNA molecules peaks 21 h after fertilization when there are 2.0 X 10(6) molecules per embryo (a 500-fold increase) and then decreases over the next 3.25 h to 1.3 million molecules per embryo. Between 24 and 82 h after fertilization the number of H1-gamma transcripts declines steadily (210 molecules per min per embryo) to reach approximately 5.4 X 10(5) H1-gamma mRNAs by 82 h postfertilization. Surprisingly, the number of late H1 mRNA molecules per embryo is greater than the number of late H2B mRNA molecules beginning at the early gastrula stage of development.