It is well established that the transcription rate of the rRNA gene is closely associated with profound alterations in the cell growth rate. Regulation of rRNA gene transcription is likely to be dependent on the dynamic conversion of the chromatin structure. Previously, we identified B23/nucleophosmin, a multifunctional nucleolar phosphoprotein, as a component of template activating factor III that remodels the chromatinlike structure of the adenovirus genome complexed with viral basic proteins. It has also been shown that B23 has histone chaperone activity. Here, we examined the effect of B23 on rRNA gene transcription. B23 was found to be associated with the rRNA gene chromatin. Small-interfering-RNA-mediated down-regulation of the B23 expression level resulted in reduction of the transcription rate of the rRNA gene. We constructed a B23 mutant termed B23⌬C, which lacks the domain essential for the histone chaperone activity and inhibited the histone binding activity of B23 in a dominant-negative manner. Expression of B23⌬C decreased rRNA gene transcription and the rate of cell proliferation. These results suggest that B23 is involved in the transcription regulation of the rRNA gene as a nucleolar histone chaperone.

PIWI-interacting RNAs (piRNAs) mediate transcriptional and post-transcriptional silencing of transposable element (TE) in animal gonads. In Drosophila ovaries, Piwi-piRNA complexes (Piwi-piRISCs) repress TE transcription by modifying the chromatin state, such as by H3K9 trimethylation. Here, we demonstrate that Piwi physically interacts with linker histone H1. Depletion of Piwi decreases H1 density at a subset of TEs, leading to their derepression. Silencing at these loci separately requires H1 and H3K9me3 and heterochromatin protein 1a (HP1a). Loss of H1 increases target loci chromatin accessibility without affecting H3K9me3 density at these loci, while loss of HP1a does not impact H1 density. Thus, Piwi-piRISCs require both H1 and HP1a to repress TEs, and the silencing is correlated with the chromatin state rather than H3K9me3 marks. These findings suggest that Piwi-piRISCs regulate the interaction of chromatin components with target loci to maintain silencing of TEs through the modulation of chromatin accessibility.

The PIWI-interacting RNA (piRNA) pathway preserves genomic integrity by repressing transposable elements (TEs) in animal germ cells. Among PIWI-clade proteins in Drosophila, Piwi transcriptionally silences its targets through interactions with cofactors, including Panoramix (Panx) and forms heterochromatin characterized by H3K9me3 and H1. Here, we identified Nxf2, a nuclear RNA export factor (NXF) variant, as a protein that forms complexes with Piwi, Panx, and p15. Panx-Nxf2-P15 complex formation is necessary in the silencing by stabilizing protein levels of Nxf2 and Panx. Notably, ectopic targeting of Nxf2 initiates co-transcriptional repression of the target reporter in a manner independent of H3K9me3 marks or H1. However, continuous silencing requires HP1a and H1. In addition, Nxf2 directly interacts with target TE transcripts in a Piwidependent manner. These findings suggest a model in which the Panx-Nxf2-P15 complex enforces the association of Piwi with target transcripts to trigger co-transcriptional repression, prior to heterochromatin formation in the nuclear piRNA pathway. Our results provide an unexpected connection between an NXF variant and small RNA-mediated co-transcriptional silencing. The EMBO JournalKensaku Murano et al F Density plots for normalized H3K9me3 ChIP-seq signals over the consensus sequence from mdg1, gypsy (targeted by Piwi-piRNA, orange), and roo (not targeted by Piwi-piRNA, blue) TEs in EGFP-, Piwi-, Panx-, and Nxf2-KD OSCs. G Boxplots as in (D) showing fold changes in the H3K9me3 levels of Piwi-piRNA-targeted and un-targeted TEs upon the indicated KD. H Northern blotting for Idefix-piRNA, traffic jam (tj)-piRNA, and esiRNA sl-1 (control) on total RNA isolated from OSCs with the indicated KD.

1The PIWI-interacting RNA (piRNA) pathway preserves genomic integrity by 2 repressing transposable elements (TEs) in animal germ cells. Among PIWI-clade 3 proteins in Drosophila, Piwi transcriptionally silences its targets through interactions 4 with cofactors, including Panoramix (Panx) and forms heterochromatin characterized by 5H3K9me3 and H1. Here, we identified Nxf2, a nuclear RNA export factor (NXF) 6 variant, as a protein that forms complexes with Piwi, Panx, and p15. Panx-Nxf2-p15 7 complex formation is necessary in the silencing by stabilizing protein levels of Nxf2 8 and Panx. Notably, ectopic targeting of Nxf2 initiates co-transcriptional repression of 9 the target reporter in a manner independent of H3K9me3 marks or H1. However, 10 continuous silencing requires HP1a and H1. In addition, Nxf2 directly interacts with 11 target TE transcripts in a Piwi-dependent manner. These findings suggest a model in 12 which the Nxf2-Panx-p15 complex enforces the association of Piwi with target 13

Zygotic genome activation (ZGA) is a critical postfertilization step that promotes totipotency and allows different cell fates to emerge in the developing embryo. MERVL (murine endogenous retrovirus-L) is transiently upregulated at the two-cell stage during ZGA. Although MERVL expression is widely used as a marker of totipotency, the role of this retrotransposon in mouse embryogenesis remains elusive. Here, we show that full-length MERVL transcripts, but not encoded retroviral proteins, are essential for accurate regulation of the host transcriptome and chromatin state during preimplantation development. Both knockdown and CRISPRi-based repression of MERVL result in embryonic lethality due to defects in differentiation and genomic stability. Furthermore, transcriptome and epigenome analysis revealed that loss of MERVL transcripts led to retention of an accessible chromatin state at, and aberrant expression of, a subset of two-cell-specific genes. Taken together, our results suggest a model in which an endogenous retrovirus plays a key role in regulating host cell fate potential.