STRUCTURALLY DIVERGENT HISTONE H1 VARIANTS IN CHROMOSOMES CONTAINING HIGHLY CONDENSED INTERPHASE CHROMATIN

Condensed and late-replicating interphase chromatin in the Dipertan in sect Chironomus contains a divergent type of histone H1 with an insert ed KAPKAP repeat that is conserved in single H1 variants of Caenorhabd itis elegans and Volvox carteri. H1 peptides comprising the insertion interact specifically with DNA. The Chironomid Glyptotendipes exhibits a corresponding correlation between the presence of condensed chromos ome sections and the appearance of a divergent H1 subtype. The centrom ere regions and other sections of Glyptotendipes barbipes chromosomes are inaccessible to immunodecoration by anti-H2B and anti-H1 antibodie s one of which is known to recognize nine different epitopes in all do mains of the H1 molecule. Microelectrophoresis of the histones from ma nually isolated unfixed centromeres revealed the presence of H1 and co re histones. H1 genes of G. barpipes were sequenced and found to belon g to two groups. H1 II and H1 III are rather similar but differ remark ably from H1 I. About 30% of the deduced amino acid residues were foun d to be unique to H1 I. Most conspicuous is the insertion, SPAKSPGR, i n H1 I that is lacking in H1 II and H1 III and at its position gives r ise to the sequence repeat SPAKSPAKSPGR. The homologous H1 I gene in G lyptotendipes salinus encodes the very similar repeat TPAKSPAKSPGR. Bo th sequences are structurally related to the KAPKAP repeat in H1 I-1 s pecific for condensed chromosome sites in Chironomus and to the SPKKSP KK repeat in sea urchin sperm H1, lie at almost the same distance from the central globular domain, and could interact with linker DNA in pa ckaging condensed chromatin.