THE 5 CLEAVAGE-STAGE (CS) HISTONES OF THE SEA-URCHIN ARE ENCODED BY AMATERNALLY EXPRESSED FAMILY OF REPLACEMENT HISTONE GENES - FUNCTIONALEQUIVALENCE OF THE CS H1 AND FROG H1M (B4) PROTEINS

The cleavage-stage (CS) histones of the sea urchin are known to be mat ernally expressed in the egg, have been implicated in chromatin remode ling of the male pronucleus following fertilization, and are the only histone variants present in embryonic chromatin up to the four-cell st age, With the help of partial peptide sequence information, we have is olated and identified CS H1, H2A, H2B, H3, and H4 cDNAs from egg poly( A)(+) mRNA of the sea urchin Psammechinus miliaris. All five CS protei ns correspond to replacement histone variants which are encoded by rep lication-independent genes containing introns, poly(A) addition signal s, and long nontranslated sequences, Transcripts of the CS histone gen es could be detected only during oogenesis and in development up to th e early blastula stage, The CS proteins, with the exception of H4, are unique histones which are distantly related in sequence to the early, late, and sperm histone subtypes of the sea urchin, In contrast, the CS H1 protein displays highest sequence homology with the H1M (B4) his tone of Xenopus laevis, Both H1 proteins are replacement histone varia nts with very similar developmental expression profiles in their respe ctive species, thus indicating that the frog H1M (B4) gene is a verteb rate homolog of the CS H1 gene, These data furthermore suggest that th e CS histones are of ancient evolutionary origin and may perform simil ar conserved functions during oogenesis and early development in diffe rent species.