Abstract
Germline chromatin undergoes dramatic remodeling events involving histone variants during the life cycle of an organism. A universal histone variant, H3.3, is incorporated at sites of active transcription throughout the cell cycle. The presence of H3.3 in chromatin indicates histone turnover, which is the energy-dependent removal of preexisting histones and replacement with new histones. H3.3 is also incorporated during decondensation of the Drosophila sperm pronucleus, indicating a direct role in chromatin remodeling upon fertilization. Here we present a system to monitor histone turnover and chromatin remodeling during Caenorhabditis elegans development by following the developmental dynamics of H3.3. We generated worm strains expressing green fluorescent protein– or yellow fluorescent protein–fused histone H3.3 proteins, HIS-71 and HIS-72. We found that H3.3 is retained in mature sperm chromatin, raising the possibility that it transmits epigenetic information via the male germline. Upon fertilization, maternal H3.3 enters both male and female pronuclei and is incorporated into paternal chromatin, apparently before the onset of embryonic transcription, suggesting that H3.3 can be incorporated independent of transcription. In early embryos, H3.3 becomes specifically depleted from primordial germ cells. Strikingly, the X chromosome becomes deficient in H3.3 during gametogenesis, indicating a low level of histone turnover. These results raise the possibility that the asymmetry in histone turnover between the X chromosome and autosomes is established during gametogenesis. H3.3 patterns are similar to patterns of H3K4 methylation in the primordial germ cells and on the X chromosome during gametogenesis, suggesting that histone turnover and modification are coupled processes. Our demonstration of dynamic H3.3 incorporation in nondividing cells provides a mechanistic basis for chromatin changes during germ cell development.
Highlights
Germ cell chromatin undergoes many dynamic remodeling events during gametogenesis and upon fertilization
To identify H3.3 genes within the C. elegans genome, we performed a tblastn search and found five high-scoring hits containing H3.3-specific residues (Figure 1A). These include four previously annotated histone genes: his-69 (E03A3.3), his-70 (E03A3.4), his-71 (F45E1.6), and his-72 (Y49E10.6). his-69 and his-70 are predicted to encode Nand C-terminally truncated proteins, respectively. his-70 may encode a protein lacking the C-terminal six to eight amino acids that was identified previously in total histone H3 purified from C. elegans [13]. his-69 and his-70 are tandem genes, and only one H3.3-like gene is present in the syntenic position in Caenorhabditis briggsae
We have introduced a system to visualize histone variant H3.3 dynamics in living C. elegans
Summary
Germ cell chromatin undergoes many dynamic remodeling events during gametogenesis and upon fertilization. The histone variants are sequentially replaced by transition proteins and protamines, resulting in highly condensed and transcriptionally inert sperm DNA (reviewed in [1]). Sperm DNA is again remodeled upon fertilization, where it is rapidly decondensed, and protamines are replaced by histones to generate transcriptionally competent chromatin. Germ cell chromatin undergoes other dynamic processes such as meiotic recombination, the formation of the XY body and establishment of genomic imprinting mediated by DNA methylation. Unlike S phase or replication-coupled (RC) histones, are expressed throughout the cell cycle and are incorporated into nucleosomes in a DNA replication– independent (RI) manner. H3.3 is incorporated during decondensation of the Drosophila sperm pronucleus, indicating a direct role in chromatin remodeling upon fertilization [11]
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