SUMMARY The histone modifying complexes PRC2 and TrxG/MLL play pivotal roles in determining the activation state of genes controlling pluripotency, lineage commitment, and cell differentiation. Long non-coding RNAs (lncRNAs) can bind to either complex, and some have been shown to act as modulators of PRC2 or TrxG/MLL activity. Here we show that the lateral mesoderm-specific lncRNA Fendrr is essential for proper heart and body wall development in the mouse. Embryos lacking Fendrr displayed upregulation of several transcription factors controlling lateral plate or cardiac mesoderm differentiation, accompanied by a drastic reduction in PRC2 occupancy along with decreased H3K27 trimethylation and/or an increase in H3K4 trimethylation at their promoters. Fendrr binds to both the PRC2 and TrxG/MLL complexes, suggesting that it acts as modulator of chromatin signatures that define gene activity. Thus, our work identifies a lncRNA that plays an essential role in fine-tuning the regulatory networks which control the fate of lateral mesoderm derivatives.

Recent work has shown that RNA polymerase (Pol) II can be recruited to and transcribe distal regulatory regions. Here we analyzed transcription initiation and elongation through genome-wide localization of Pol II, general transcription factors (GTFs) and active chromatin in developing T cells. We show that Pol II and GTFs are recruited to known T cell-specific enhancers. We extend this observation to many new putative enhancers, a majority of which can be transcribed with or without polyadenylation. Importantly, we also identify genomic features called transcriptional initiation platforms (TIPs) that are characterized by large areas of Pol II and GTF recruitment at promoters, intergenic and intragenic regions. TIPs show variable widths (0.4-10 kb) and correlate with high CpG content and increased tissue specificity at promoters. Finally, we also report differential recruitment of TFIID and other GTFs at promoters and enhancers. Overall, we propose that TIPs represent important new regulatory hallmarks of the genome.

Combinations of post-translational histone modifications shape the chromatin landscape during cell development in eukaryotes. However, little is known about the modifications exactly delineating functionally engaged regulatory elements. For example, although histone H3 lysine 4 mono-methylation (H3K4me1) indicates the presence of transcriptional gene enhancers, it does not provide clearcut information about their actual position and stagespecific activity. Histone marks were, therefore, studied here at genomic loci differentially expressed in early stages of T-lymphocyte development. The concomitant presence of the three H3K4 methylation states (H3K4me1/2/3) was found to clearly reflect the activity of bona fide T-cell gene enhancers. Globally, gain or loss of H3K4me2/3 at distal genomic regions correlated with, respectively, the induction or the repression of associated genes during T-cell development. In the Tcrb gene enhancer, the H3K4me3-to-H3K4me1 ratio decreases with the enhancer's strength. Lastly, enhancer association of RNApolymerase II (Pol II) correlated with the presence of H3K4me3 and Pol II accumulation resulted in local increase of H3K4me3. Our results suggest the existence of functional links between Pol II occupancy, H3K4me3 enrichment and enhancer activity.