Emily L Naden

In contrast to the well-understood role of core histones in DNA packaging, the function of the linker histone (H1) remains enigmatic. Challenging the prevailing view that linker histones are a general feature of heterochromatin, here we show a critical requirement for H1 in Polycomb repressive complex 2 (PRC2) function. A CRISPR-Cas9 genetic screen using a fluorescent PRC2 reporter identified an essential role for the poorly characterized gene CRAMP1 in PRC2-mediated repression. CRAMP1 localizes to the promoters of expressed H1 genes and positively regulates their transcription. CRAMP1 ablation simultaneously depletes all linker histones, which results in selective decompaction of H3K27me3-marked loci and derepression of PRC2 target genes without concomitant loss of PRC2 occupancy or enzymatic activity. Strikingly, we find that linker histones preferentially localize to genomic loci marked by H3K27me3 across diverse cell types and organisms. Altogether, these data demonstrate a prominent role for linker histones in epigenetic repression by PRC2. • A genome-wide CRISPR screen reveals CRAMP1 is required for PRC2 repression • CRAMP1 binds linker histone genes and drives their expression • Linker histones are enriched at H3K27me3-marked genomic loci • Linker histone insufficiency following CRAMP1 ablation abrogates PRC2 repression Challenging the prevailing view that H1 linker histones represent a general feature of repressed chromatin, Matthews et al. show a specific requirement for H1 in epigenetic repression by PRC2. Ablation of the H1 activator CRAMP1 results in linker histone insufficiency and derepression of PRC2 target genes.

The tight control of the mitotic phase of differentiation is crucial to prevent tumourigenesis while securing tissue homeostasis. In the Drosophila female germline, differentiation involves precisely four mitotic divisions, and accumulating evidence suggests that bag of marbles (bam), the initiator of differentiation, is also involved in controlling the number of divisions. To test this hypothesis, we depleted Bam from differentiating cells and found a reduced number of mitotic divisions. We examined the regulation of Bam using RNA imaging methods and found that the bam 3' UTR conveys instability to the transcript in the eight-cell cyst and early 16-cell cyst. We show that the RNA-binding protein Rbp9 is responsible for timing bam mRNA decay. Rbp9 itself is part of a sequential cascade of RNA-binding proteins activated downstream of Bam, and we show that it is regulated through a change in transcription start site, driven by Rbfox1. Altogether, we propose a model in which Bam expression at the beginning of differentiation initiates a series of events that eventually terminates the Bam expression domain.

Abstract The tight control of the mitotic phase of differentiation is crucial to prevent tumourigenesis while securing tissue homeostasis. In the Drosophila female germline, differentiation involves precisely four mitotic divisions, and accumulating evidence suggests that bag-of-marbles ( bam ), the initiator of differentiation, is also involved in controlling the number of divisions. To test this hypothesis, we depleted Bam from differentiating cells and found a reduced number of mitotic divisions. We examined the regulation of Bam using RNA imaging methods and found that the bam 39 UTR conveys instability to the transcript in the 8-cell cyst and early 16-cell cyst. We show that the RNA binding protein, Rbp9, is responsible for timing bam mRNA decay. Rbp9 itself is part of a sequential cascade of RNA binding proteins activated downstream of Bam, and we show that it is regulated through a change in transcription start site, driven by Rbfox1. Altogether, we propose a model in which Bam expression at the dawn of differentiation initiates a series of events that eventually terminates the Bam expression domain.