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. 2009 Nov 17;19(21):1816-20.
doi: 10.1016/j.cub.2009.09.021. Epub 2009 Sep 24.

Transcriptional and developmental functions of the H3.3 histone variant in Drosophila

Akiko Sakai  1 , Brian E Schwartz, Sara Goldstein, Kami Ahmad
Affiliations

Transcriptional and developmental functions of the H3.3 histone variant in Drosophila

Akiko Sakai et al. Curr Biol. .

Abstract

Changes in chromatin composition accompany cellular differentiation in eukaryotes. Although bulk chromatin is duplicated during DNA replication, replication-independent (RI) nucleosome replacement occurs in transcriptionally active chromatin and during specific developmental transitions where the genome is repackaged. In most animals, replacement uses the conserved H3.3 histone variant, but the functions of this variant have not been defined. Using mutations for the two H3.3 genes in Drosophila, we identify widespread transcriptional defects in H3.3-deficient animals. We show that mutant animals compensate for the lack of H3.3 in two ways: they upregulate the expression of the major histone H3 genes, and they maintain chromatin structure by using H3 protein for RI nucleosome replacement at active genes. Rescue experiments show that increased expression of H3 is sufficient to relieve transcriptional defects. In contrast, H3.3 is essential for male fertility, and germline cells specifically require the histone variant. Defects without H3.3 first occur around meiosis, resulting in a failure to condense, segregate, and reorganize chromatin. Rescue experiments with mutated transgenes demonstrate that H3.3-specific residues involved in RI nucleosome assembly-but not major histone modification sites-are required for male fertility. Our results imply that the H3.3 variant plays an essential role in chromatin transitions in the male germline.

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Figures

Figure 1
Figure 1. The major H3 histone can be used for RI nucleosome assembly in H3.3-deficient animals
(A–C) Deposition of newly-produced histone-GFP protein at induced Hsp70 genes in polytene chromosomes. Larvae carrying inducible histone-GFP genes were heat shocked for 1 hour at 37°, allowed to recover for 2 hours, and then stained for GFP (green) and DNA (red). The genotypes of larvae are indicated. (A) Wild-type larvae carrying an inducible H3-GFP construct show no RI deposition, while (B) wild-type larvae carrying an inducible H3.3-GFP construct show efficient deposition of GFP at the induced Hsp70 genes. (E) Double-null larvae carrying an inducible H3-GFP construct show deposition of the tagged histone.
Figure 2
Figure 2. Rescue of transcriptional mis-regulation by H3.3 or H3 histones
(A) Comparison of transcriptional changes in adult males carrying rescue transgenes. The x-axis displays fold-changes in gene expression between H3.3-deficient males and males carrying the His3.3A rescue transgene, and the y-axis displays fold-changes between H3.3-deficient males and males carrying the His3.3AH3 transgene. Genes are color-coded in quantiles of transcript abundance. Genes with equal expression in His3.3A and His3.3AH3 –rescued males are expected to lie on the diagonal. Expression of most genes is rescued by either transgene. Genes not rescued by H3 expression are listed in Supplementary Table 2. (B) Expression of histone genes in H3.3-deficient and rescued adult males. The relative change in expression between wild-type-rescued animals and null animals (green), and between wild-type-rescued animals and H3-rescued animals (red) is shown). The mutated His3.3A and His3.3B genes are not shown.
Figure 3
Figure 3. Condensation and segregation defects in H3.3-deficient meiotic cells
(A) Meiotic chromosome spreads in wild-type (top) and H3.3-deficient (bottom) testes. Testes were stained with anti-H3K4trime antibodies (green) to distinguish euchromatic and heterochromatic regions and orient chromosomes. DAPI-stained DNA is in red. In Metaphase I, chromosomes from wild-type testes are compacted and distinct, while those from H3.3-deficient testes are poorly resolved. In Anaphase I in wild-type testes, similar amounts of chromatin segregate to each pole. In H3.3-deficient testes, unequal numbers of chromosomes segregate between the two poles of division, and chromosomes are trapped between the poles. In Anaphase II in wildtype testes, sister chromatids segregate equally between the poles of division (four products from a single meiosis are shown). In Anaphase II in H3.3-deficient testes, chromatin bridges span the two poles of division. (B–G) Modification sites of H3.3 are not required for male germline function. Phase-contrast microscopy of post-meiotic round spermatid nuclei. Genotypes and mutant rescue constructs are indicated. (B) In wild-type testes, each nucleus contains similar amounts of chromatin, and a single nuclear body (phase-dark spot). (C) In H3.3-deficient testes, some round nuclei are larger or smaller than wild-type (yellow arrow indicates a micronucleus), resulting from aberrant segregation of chromosomes in meiosis. The nuclear body in many nuclei is fragmented into numerous freckles. (D) Double-null mutants carrying a His3.3AH3 rescue transgene show large and small nuclei (yellow arrow indicates a micronucleus), and fragmented nuclear bodies. (E) Double-null mutants carrying a His3.3AK4R rescue transgene show equal-sized nuclei and a single nuclear body in each nucleus. (F) Double-null mutants carrying a His3.3AK9R rescue transgene show equal-sized nuclei and a single nuclear body in each nucleus. (G) Double-null mutants carrying a His3.3AS31A rescue transgene show equal-sized nuclei and a single nuclear body in each nucleus.
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