Chromatin remodeling complexes regulate genome architecture in Arabidopsis.

In eukaryotes, three-dimensional (3D) chromatin architecture maintains genome stability and is important in regulating gene transcription. However, little is known about the mechanisms by which diverse ATP-dependent chromatin remodeling complexes regulate the 3D chromatin structure in plants. We examined the 3D chromatin structure within the ATPase subunit of the SWI/SNF, ISWI, INO80, and CHD remodeling complexes in wild-type (WT) and mutant Arabidopsis thaliana plants by combining high-throughput sequencing with in situ Hi-C, the enrichment of histone marks, nucleosome density, and gene expression. We found that compartment regions switched and compartmental strength was significantly weakened in all four enzyme mutants. Chromatin remodeling complexes differentially regulated the nucleosome distribution pattern and density within the switching compartments. Alterations of nucleosome distribution pattern and density were associated with a reduction in H3K27me3 levels in the chromatin remodeling enzyme mutants and led to compartment switching. Our data show that chromatin remodeling complexes regulate the linear nucleosome distribution pattern and density to promote H3K27me3 deposition, which in turn regulates 3D chromatin structure.

New class of transcription factors controls flagellar assembly by recruiting RNA polymerase II in Chlamydomonas.

Cells have developed regulatory mechanisms that underlie flagellar assembly and maintenance, including the transcriptional regulation of flagellar genes, an initial step for making flagella. Although transcriptional regulation of flagellar gene expression is required for flagellar assembly in Chlamydomonas, no transcription factor that regulates the transcription of flagellar genes has been identified. We report that X chromosome-associated protein 5 (XAP5) acts as a transcription factor to regulate flagellar assembly in Chlamydomonas While XAP5 proteins are evolutionarily conserved across diverse organisms and play vital roles in diverse biological processes, nothing is known about the biochemical function of any member of this important protein family. Our data show that loss of XAP5 leads to defects in flagellar assembly. Posttranslational modifications of XAP5 track flagellar length during flagellar assembly, suggesting that cells possess a feedback system that modulates modifications to XAP5. Notably, XAP5 regulates flagellar gene expression via directly binding to a motif containing a CTGGGGTG-core. Furthermore, recruitment of RNA polymerase II (Pol II) machinery for transcriptional activation depends on the activities of XAP5. Our data demonstrate that, through recruitment of Pol II, XAP5 defines a class of transcription factors for transcriptional regulation of ciliary genes. This work provides insights into the biochemical function of the XAP5 family and the fundamental biology of the flagellar assembly, which enhance our understanding of the signaling and functions of flagella.

Interior circular RNA.

Formed by back splicing or back fusion of linear RNAs, circular RNAs (circRNAs) constitute a new class of non-coding RNAs of eukaryotes. Recent studies reveal a spliceosome-dependent biogenesis of circRNAs where circRNAs arise at the intron-exon junctions of mRNAs. In this study, using a novel de novo identification method, we show that circRNAs can originate from the interior regions of exons, introns, and intergenic transcripts in human, mouse and rice, which were referred to as interior circRNAs (i-circRNAs). Many i-circRNAs have some remarkable characteristics: multiple i-circRNAs may arise from the same genomic locus; their back fusion points may not be associated with the AG/GT splicing sites, but rather a new pair of motif AC/CT, their back fusion points are adjacent to complementary sequences; and they may circulate on short homologous sequences. We validated several i-circRNAs in HeLa cells by Polymerase Chain Reaction followed by Sanger sequencing. Our results combined showed that i-circRNAs are bona fide circRNAs, indicated novel biogenesis pathways independent of the splicing apparatus, and expanded our understanding of the origin, diversity, and complexity of circRNAs.

H2AK121ub in Arabidopsis associates with a less accessible chromatin state at transcriptional regulation hotspots.

Although it is well established that the Polycomb Group (PcG) complexes maintain gene repression through the incorporation of H2AK121ub and H3K27me3, little is known about the effect of these modifications on chromatin accessibility, which is fundamental to understand PcG function. Here, by integrating chromatin accessibility, histone marks and expression analyses in different Arabidopsis PcG mutants, we show that PcG function regulates chromatin accessibility. We find that H2AK121ub is associated with a less accessible but still permissive chromatin at transcriptional regulation hotspots. Accessibility is further reduced by EMF1 acting in collaboration with PRC2 activity. Consequently, H2AK121ub/H3K27me3 marks are linked to inaccessible although responsive chromatin. In contrast, only-H3K27me3-marked chromatin is less responsive, indicating that H2AK121ub-marked hotspots are required for transcriptional responses. Nevertheless, despite the loss of PcG activities leads to increased chromatin accessibility, this is not necessarily accompanied by transcriptional activation, indicating that accessible chromatin is not always predictive of gene expression.