RNA interference with one of the eight Caenorhabditis elegans linker histone genes triggers desilencing of a repetitive transgene and developmental defects in the hermaphrodite germ line. These characteristics are similar to the phenotype of the C. elegans Polycomb group genes mes-2, mes-3, mes-4, and mes-6 (M. A. Jedrusik and E. Schulze, Development 128:1069-1080, 2001; I. Korf, Y. Fan, and S. Strome, Development 125:2469-2478, 1998). These Polycomb group proteins contribute to germ line-specific chromatin modifications. Using a his-24 deletion mutant and an isoform-specific antibody, we characterized the role of his-24 in C. elegans germ line development. We describe an unexpected cytoplasmic retention of HIS-24 in peculiar granular structures. This phenomenon is confined to the developing germ lines of both sexes. It is strictly dependent on the activities of the chromatinmodifying genes mes-2, mes-3, mes-4, and mes-6, as well as on the C. elegans sirtuin gene sir-2.1. A temperature shift experiment with a mes-3(ts) mutant revealed that mes gene activity is required in a time window ranging from L3 to the early L4 stage before the onset of meiosis. We find that the his-24(ok1024) mutant germ line is characterized by an increased level of the activating H3K4 methylation mark concomitant with a decrease of the repressive H3K9 methylation. In the germ line of his-24(ok1024) mes-3(bn35) double mutant animals, the repressive H3K27 methylation is more reduced than in the respective mes single mutant. These observations distinguish his-24 as an unusual element in the developmental regulation of germ line chromatin structure in C. elegans.Linker histones are highly abundant chromatin proteins that bind to the elementary structural unit of the chromatin, the nucleosome. Our previous work (9) characterized the Caenorhabditis elegans linker histone variant gene his-24 (H1.1) as a gene involved in the control of hermaphrodite germ line development. Using RNA interference (RNAi), desilencing of a repetitive transgene in the germ lines of both sexes had been shown. Additionally a low-penetrant cytological gonad phenotype occurred, where the germ line substantially lacked proliferation and differentiation. This cytological phenotype was observed in hermaphrodites but not in males. The combination of both observations related his-24 to the phenotype of the C. elegans Polycomb group genes, mes-2, mes-3, mes-4, and mes-6 (18; for reviews, see references 25 and 27). The precise gonadal expression pattern of HIS-24, its general mode of action, and its specific functional relationship to the mes genes remained unclear. No germ line phenotype of a linker histone mutant has been reported for mammals so far, although the mouse linker histone complement has recently been recognized as essential for embryogenesis (6). The present view on linker histones in general describes them as highly dynamic chromatin components. They are considered to be dispensable in single-cell eukaryotes (4, 16).The C. elegans SET domain histone methyl trans...

Linker histones are nonessential for the life of single-celled eukaryotes. Linker histones, however, can be important components of specific developmental programs in multicellular animals and plants. For Caenorhabditis elegans a single linker histone variant (H1.1) is essential in a chromatin silencing process which is crucial for the proliferation and differentiation of the hermaphrodite germ line. In this study we analyzed the whole linker histone complement of C. elegans by telomeric position effect variegation in budding yeast. In this assay an indicator gene (URA3) placed close to the repressive telomeric chromatin structure is subject to epigenetically inherited gene inactivation. Just one out of seven C. elegans linker histones (H1.1) was able to enhance the telomeric position effect in budding yeast. Since these results reflect the biological function of H1.1 in C. elegans, we suggest that chromatin silencing in C. elegans is governed by molecular mechanisms related to the telomere-dependent silencing in budding yeast. We confirmed this hypothesis by testing C. elegans homologs of three yeast genes which are established modifiers of the yeast telomeric chromatin structure (SIR2, SET1, and RAD17) for their influence on repeat-dependent transgene silencing for C. elegans.Linker histones are highly abundant eukaryotic chromatin proteins. They bind to the nucleosomes, forming the 30-nm chromatin fiber. The biological function of linker histones is not sufficiently clear. In contrast to the core histones which are essential for eukaryotic life (17, 25), linker histones are dispensable in single-celled organisms, as shown by knockouts performed with Tetrahymena (40), yeast (13,35), and in simple multicellular fungi such as Aspergillus (36) and Ascobolus (8). For multicellular eukaryotes, linker histones usually exist as a set of relatively divergent protein variants. The occurrence of a typical linker histone gene family is correlated with multicellularity in plants as well as in animals. For a long time it has been considered that different linker histone variants could fulfill different biochemical functions, but no canonical test has been established to investigate this. Only recently it has become apparent that individual linker histone variants can specifically contribute to essential aspects of multicellular life, like cell differentiation and development (for a review, see reference 23). We use Caenorhabditis elegans as a model system to investigate this question. C. elegans possesses eight different linker histone variants, the same number as for humans (3, 4, 47) or mice (12), and it allows dissection of the functions of individual linker histone genes with RNA interference (RNIi) and cytological techniques. In previous work we characterized the function of the major histone H1 variant in C. elegans, H1.1 (19). This protein is essential for the germ line-specific chromatin silencing and consequently also for hermaphrodite fertility. The phenotype of H1.1 depletion is very similar to the phenotype of ...

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In remarkable contrast to somatic cells, the germline of the nematode Caenorhabditis elegans efficiently silences transgenic DNA. The molecular mechanisms responsible for this have been shown to implicate chromatin proteins encoded by the mes genes (Kelly, W. G. and Fire, A. (1998) Development 125, 2451–2456), of which two are the C. elegans homologs of Polycomb Group gene transcriptional repressors. We have analyzed the contribution of the histone H1 gene family to this specific aspect of germ cells in C. elegans. We show with isotype-specific double stranded RNA-mediated interference (RNAi) that a single member of this gene family (H1.1) is essential for the repression of a silenced reporter-transgene in the germline of hermaphrodites and males, whereas no change is found in the somatic expression of this reporter. Additionally, RNA-mediated interference with H1.1 gene expression can cause a phenotype with severe affection of germline proliferation and differentiation in the hermaphrodite, and even sterility (5%-11% penetrance). These and further features observed in histone H1.1 RNAi experiments are also characteristic of the mes phenotype (Garvin, C., Holdeman, R. and Strome, S. (1998) Genetics 148, 167–185), which is believed to result from the desilencing of genes required for somatic differentiation in the germline. Our observations therefore support this interpretation of the mes phenotype and they identify a single histone H1 isoform (H1.1) as a new component specifically involved in chromatin silencing in the germline of C. elegans.

The histone H1 complement of Caenorhabditis elegans contains a single unusual protein, H1.X. Although H1.X possesses the globular domain and the canonical three-domain structure of linker histones, the amino acid composition of H1.X is distinctly different from conventional linker histones in both terminal domains. We have characterized H1.X in C. elegans by antibody labeling, green fluorescent protein fusion protein expression and RNA interference. Unlike normal linker histones, H1.X is a cytoplasmic as well as a nuclear protein and is not associated with chromosomes. H1.X is most prominently expressed in the marginal cells of the pharynx and is associated with a peculiar cytoplasmic cytoskeletal structure therein, the tonofilaments. Additionally H1.X::GFP is expressed in the cytoplasm of body and vulva muscle cells, neurons, excretory cells and in the nucleoli of embryonic blastomeres and adult gut cells. RNA interference with H1.X results in uncoordinated and egg laying defective animals, as well as in a longitudinally enlarged pharynx. These phenotypes indicate a cytoplasmic role of H1.X in muscle growth and muscle function.