Prdm3 and Prdm16 are H3K9me1 methyltransferases required for mammalian heterochromatin integrity.

Heterochromatin serves important functions, protecting genome integrity and stabilizing gene expression programs. Although the Suv39h methyltransferases (KMTs) are known to ensure pericentric H3K9me3 methylation, the mechanisms that initiate and maintain mammalian heterochromatin organization remain elusive. We developed a biochemical assay and used in vivo analyses in mouse embryonic fibroblasts to identify Prdm3 and Prdm16 as redundant H3K9me1-specific KMTs that direct cytoplasmic H3K9me1 methylation. The H3K9me1 is converted in the nucleus to H3K9me3 by the Suv39h enzymes to reinforce heterochromatin. Simultaneous depletion of Prdm3 and Prdm16 abrogates H3K9me1 methylation, prevents Suv39h-dependent H3K9me3 trimethylation, and derepresses major satellite transcription. Most strikingly, DNA-FISH and electron microscopy reveal that combined impairment of Prdm3 and Prdm16 results in disintegration of heterochromatic foci and disruption of the nuclear lamina. Our data identify Prdm3 and Prdm16 as H3K9me1 methyltransferases and expose a functional framework in which anchoring to the nuclear periphery helps maintain the integrity of mammalian heterochromatin.

Estrogen-dependent control and cell-to-cell variability of transcriptional bursting.

Cellular decision-making and environmental adaptation are dependent upon a heterogeneous response of gene expression to external cues. Heterogeneity arises in transcription from random switching between transcriptionally active and inactive states, resulting in bursts of RNA synthesis. Furthermore, the cellular state influences the competency of transcription, thereby globally affecting gene expression in a cell-specific manner. We determined how external stimuli interplay with cellular state to modulate the kinetics of bursting. To this end, single-cell dynamics of nascent transcripts were monitored at the endogenous estrogen-responsive GREB1 locus. Stochastic modeling of gene expression implicated a two-state promoter model in which the estrogen stimulus modulates the frequency of transcriptional bursting. The cellular state affects transcriptional dynamics by altering initiation and elongation kinetics and acts globally, as GREB1 alleles in the same cell correlate in their transcriptional output. Our results suggest that cellular state strongly affects the first step of the central dogma of gene expression, to promote heterogeneity in the transcriptional output of isogenic cells.

A C-terminal silencing domain in GW182 is essential for miRNA function.

Proteins of the GW182 family are essential for miRNA-mediated gene silencing in animal cells; they interact with Argonaute proteins (AGOs) and are required for both the translational repression and mRNA degradation mediated by miRNAs. To gain insight into the role of the GW182-AGO1 interaction in silencing, we generated protein mutants that do not interact and tested them in complementation assays. We show that silencing of miRNA targets requires the N-terminal domain of GW182, which interacts with AGO1 through multiple glycine-tryptophan (GW)-repeats. Indeed, a GW182 mutant that does not interact with AGO1 cannot rescue silencing in cells depleted of endogenous GW182. Conversely, silencing is impaired by mutations in AGO1 that strongly reduce the interaction with GW182 but not with miRNAs. We further show that a GW182 mutant that does not localize to P-bodies but interacts with AGO1 rescues silencing in GW182-depleted cells, even though in these cells, AGO1 also fails to localize to P-bodies. Finally, we show that in addition to the N-terminal AGO1-binding domain, the middle and C-terminal regions of GW182 (referred to as the bipartite silencing domain) are essential for silencing. Together our results indicate that miRNA silencing in animal cells is mediated by AGO1 in complex with GW182, and that P-body localization is not required for silencing.

Direct observation of frequency modulated transcription in single cells using light activation.

Single-cell analysis has revealed that transcription is dynamic and stochastic, but tools are lacking that can determine the mechanism operating at a single gene. Here we utilize single-molecule observations of RNA in fixed and living cells to develop a single-cell model of steroid-receptor mediated gene activation. We determine that steroids drive mRNA synthesis by frequency modulation of transcription. This digital behavior in single cells gives rise to the well-known analog dose response across the population. To test this model, we developed a light-activation technology to turn on a single steroid-responsive gene and follow dynamic synthesis of RNA from the activated locus. DOI:http://dx.doi.org/10.7554/eLife.00750.001.