Isolation, Characterization, and Expression of the Gene Encoding the Late Histone Subtype H1-Gamma of the Sea Urchin Strongylocentrotus purpuratus

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.