The tumour suppressor brain tumour (Brat) regulates linker histone dBigH1 expression in the Drosophila female germline and the early embryo

Paula Climent-Cantó,Fernando Azorín,Imre M Boros,Albert Carbonell,Laszlo Henn,Salvador Pérez-Montero,Srividya Tamirisa

doi:10.1098/rsob.200408

Abstract

Linker histones H1 are essential chromatin components that exist as multiple developmentally regulated variants. In metazoans, specific H1s are expressed during germline development in a tightly regulated manner. However, the mechanisms governing their stage-dependent expression are poorly understood. Here, we address this question in Drosophila, which encodes for a single germline-specific dBigH1 linker histone. We show that during female germline lineage differentiation, dBigH1 is expressed in germ stem cells and cystoblasts, becomes silenced during transit-amplifying (TA) cystocytes divisions to resume expression after proliferation stops and differentiation starts, when it progressively accumulates in the oocyte. We find that dBigH1 silencing during TA divisions is post-transcriptional and depends on the tumour suppressor Brain tumour (Brat), an essential RNA-binding protein that regulates mRNA translation and stability. Like other oocyte-specific variants, dBigH1 is maternally expressed during early embryogenesis until it is replaced by somatic dH1 at the maternal-to-zygotic transition (MZT). Brat also mediates dBigH1 silencing at MZT. Finally, we discuss the situation in testes, where Brat is not expressed, but dBigH1 is translationally silenced too.

Highlights

Linker histones H1 constitute a conserved family of chromosomal proteins that bind nucleosomes and play central roles in the regulation of chromatin structure and function
We report that dBigH1 silencing in cystocytes depends on the tumour 2 suppressor Brain tumour (Brat), a post-transcriptional regulator that is expressed in cystocytes and represses translation of germ stem cell (GSC) maintenance factors [40,41]
Fly stocks and genetic procedures w1118, nos-Gal4::VP16 [42] and Df(2 L)TE37C-7 were obtained from Bloomington Drosophila Stock Center (BDSC). bratRNAi and bamRNAi correspond to stocks 28 590 (y[1] v[1]; P{y[+t7.7] v[+t1.8] = TRiP.HM05078}attP2) and 33 631 (y[1] v[1]; P{y[+ t7.7] v[+t1.8] = TRiP.HMS00029}attP2) from BDSC, respectively. dBigH1NTSOP CRISPR/CAS9 mutant is described in [43]. bamP-bam::GFP and bamP-GFP [44] were a gift from Dr M

Summary

Introduction

Linker histones H1 constitute a conserved family of chromosomal proteins that bind nucleosomes and play central roles in the regulation of chromatin structure and function. Metazoan species usually contain multiple H1 variants that show differential patterns of expression during development and differentiation. In this regard, a conserved feature in metazoans is the presence of germlinespecific variants that replace somatic H1s in germ cells (reviewed in [1]). Female- and male-specific variants have been described. Female-specific variants have been described in the zebrafish (H1M) [16,17] and echiura (H1M) [18]. Drosophila encodes for a single germlinespecific linker histone, dBigH1, that is expressed in both the male and the female germline [19]. Female-specific H1s usually persist during embryo development until the zygotic genome is activated at MZT [8,10,11,12,16,17,18,19,20,21]

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Conclusion