ISOLATION AND AMINO-ACID-SEQUENCE ANALYSIS REVEAL AN ANCIENT EVOLUTIONARY ORIGIN OF THE CLEAVAGE STAGE (CS) HISTONES OF THE SEA-URCHIN

The cleavage stage (CS) H1, H2A, and H2B histones of the sea urchin, w hich have previously been identified by their distinct electrophoretic mobility on Triton/acid/urea gels, are known to be maternally express ed during oogenesis and have been implicated in chromatin remodeling o f the male pronucleus following fertilization. Here, we describe the i solation of these three CS histones by reverse-phase HPLC chromatograp hy. Moreover, a novel CS H3 protein was identified by the same purific ation procedure, A low incorporation of radioactive amino acids into t he CS histones during early development revealed that the bulk of thes e proteins in the blastula embryo are derived from the maternal pool o f the egg. Amino acid analysis, together with the previously described electrophoretic mobilities, unequivocally identified the purified pro teins as CS histones. Peptide sequence analysis confirmed the novel na ture of the CS variants as they are distantly related to the early, la te, and sperm histone subtypes of the sea urchin. The CS H1 protein di splays highest sequence similarity with the H1M (B4) histone of Xenopu s laevis, indicating that the frog H1M protein may be a vertebrate hom ologue of the CS H1 histone. These data suggest an ancient evolutionar y origin and wide distribution of the CS histone variants.