Structural and functional differences between histone H1 sequence variants with differential intranuclear distribution.

Abstract

The chromatin of most cell types contains several different sequence variants of histone Hi. The functional role of this heterogeneity is not known. In the larval tissues ofthe midge, Chironomus thummi, there are Hi variants of two types. Hi 11-1, Hi 11-2, and Hi rI-1 have similar amino acid sequences and appear uniformly distributed in polytene interphase chromosomes. The total number of gene copies per genome for this type of Hi histones is about 40 inC. th. thummi and 50-60 in C. th. piger. In contrast, histone Hi I-1 is encoded by a single copy gene in C. th. thummi and by two to four genes in C. th. piger. It has a divergent structure and is found only in a limited number of condensed chromosome sites. The N-terminal domain of Hi I-1 contains an insertion that is lacking in the other Hi variants and that is part of a variantspecific bipartite sequence Lys-Ala-Pro-Lys-Ala-Pro-Xaal0- Chironomus thummi. Several of the Hi variants of this species appear to be present throughout the polytene chromosomes, whereas Hi I-1, the variant with the most diverging structure (9), was detected only in a limited number of chromosome bands. Many of these bands replicate late in S-phase (10) and contain repetitive DNA (11, 12). They are more frequent in C. th. thummi than in C. th. piger. Accordingly, Hi I-1 was found to account for about 20%o of total Hi in larvae of C. th. piger and for about 30% in larvae of C. th. thummi (8).The demonstration within one nucleus of differences between chromosome sites in their Hi subtype content makes C. thummi a model organism for studying possible functions of Hi heterogeneity. Therefore, we have determined and compared the sequences of the different Hi histone genes of C. thummi in a search for structural features that are peculiar to Hi I-1 and could be involved in establishing a particular subtype of chromatin. We find that Hi I-1, in contrast to the other Hi variants, contains a novel sequence motif that is not known from other proteins but is also present in single Hi variants of other, evolutionarily distant organisms. An N-terminal peptide of Hi I-1 that includes this motif also differs from the homologous region in another Hi variant of C. thummi by competing with the drug Hoechst 33258 for binding to the minor groove of the DNA double helix. Interaction of the subtype-specific motif with linker DNA may play a role in intranuclear targeting of Hi I-1 or in establishing a condensed type of chromatin, or in both. MATERIALS AND METHODSClone Isolation and DNA Sequencing. Genomic clones containing histone gene sequences were identified in a C. th. thummi library (13) 2481The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

Subnuclear distribution of the entire complement of linker histone variants in Arabidopsis thaliana

“…An antiserum reacting with the remaining histone variants, encoded by 40 genes, stains chromosomes evenly. It was subsequently found that H1-I, but not H1-II, is able to compete with Hoechst 33258 for binding to the DNA minor groove (Schulze et al 1993), demonstrating divergent DNA interactions. In rat cells (Breneman et al 1993) and human cells (Banchev et al 1988), H1A and H1B bind throughout nuclei, in a pattern identical to DAPI staining, and the replacement histone, H1°, binds to distinct spots in the nuclei, often colocalizing with nucleoli.…”

The Histone H1 Genes of the Dipteran Insect, Chironomus thummi, Fall under Two Divergent Classes and Encode Proteins with Distinct Intranuclear Distribution and Potentially Different Functions

“…A structural basis for a higher degree of packaging of the nucleosome chain could be provided by a novel sequence motif, KAPKAPKAPKSPKA-X,,,-KVA, that is inserted in the N-terminal domain of HI 1-1, but is lacking in the other HI variants of C. thummi, H1 11-1, H1 11-2, and H1 111-1 (Schulze et al, 1993).…”

Linker histone subtypes and their allelic variants

“…Further observations on the distribution of H1 histone subtypes in chromatin indicated that they may differ in functional characteristics. Histone H1 subtype I‐1 (Mohr et al, 1989) enriched both at centromeres and a limited number of other bands in dipterian chromosomes has been identified (Schulze et al, 1993) to contain a specific amino acid insertion located in the NtD (N‐terminal domain), a repeated sequence motif of which is also present at the same distance from the globular domain in the histone H1 from a nematode C. elegans and green alga Volvox carteri . Furthermore, the histone H1 subtype (H1.1) identified in C. elegans was found to affect germline proliferation and differentiation by acting as chromatin silencer (Jedrusik and Schulze, 2001).…”