Histones of sea urchin embryos. Transients in transcription, translation, and the composition of chromatin

A previously unidentified histone, H1_x, is encoded by a messenger RNA that is distinct from H1_m mRNA. There is a rapid rise in the H1_x:H1_m mRNA ratio at the late eight-cell stage, just preceding the unequal fourth cleavage. This rise is blocked by actinomycin D. A similar rise in the ratio of H1 histones synthesized in vivo and incorporated into chromatin occurs at the same time. Late in the 16-cell stage, the H1_x:H1_m mRNA ratio falls to the oocyte ("maternal") level, and so does the ratio of H1 histones incorporated into chromatin. H1 synthesis was examined in the three different cell types of the 16-cell stage embryo, and differences in the relative synthesis of the two H1 histones were consistently observed. The divergence of histone synthesis pattern among the three cell types may be due to a divergence of cell cycle parameters following the fourth cleavage. These results have implications for the transcriptional regulation of histone synthesis and for the possibility of diverging chromatin composition and structure in very early development, at the time when determination is in progress.