Induction of epigenetic variation in Arabidopsis by over-expression of DNA METHYLTRANSFERASE1 (MET1)

Epigenetic marks such as DNA methylation and histone modification can vary among plant accessions creating epi-alleles with different levels of expression competence. Mutations in epigenetic pathway functions are powerful tools to induce epigenetic variation. As an alternative approach, we investigated the potential of over-expressing an epigenetic function, using DNA METHYLTRANSFERASE1 (MET1) for proof-of-concept. In Arabidopsis thaliana, MET1 controls maintenance of cytosine methylation at symmetrical CG positions. At some loci, which contain dense DNA methylation in CG- and non-CG context, loss of MET1 causes joint loss of all cytosines methylation marks. We find that over-expression of both catalytically active and inactive versions of MET1 stochastically generates new epi-alleles at loci encoding transposable elements, non-coding RNAs and proteins, which results for most loci in an increase in expression. Individual transformants share some common phenotypes and genes with altered gene expression. Altered expression states can be transmitted to the next generation, which does not require the continuous presence of the MET1 transgene. Long-term stability and epigenetic features differ for individual loci. Our data show that over-expression of MET1, and potentially of other genes encoding epigenetic factors, offers an alternative strategy to identify epigenetic target genes and to create novel epi-alleles

Upon heat stress processing of ribosomal RNA precursors into mature rRNAs is compromised after cleavage at primary P site in Arabidopsis thaliana

International audienceTranscription and processing of 45S rRNAs in the nucleolus are keystones of ribosome biogenesis. While these processes are severely impacted by stress conditions in multiple species, primarily upon heat exposure, we lack information about the molecular mechanisms allowing sessile organisms without a temperature-control system, like plants, to cope with such circumstances. We show that heat stress disturbs nucleolar structure, inhibits pre-rRNA processing and provokes imbalanced ribosome profiles in Arabidopsis thaliana plants. Notably, the accuracy of transcription initiation and cleavage at the primary P site in the 5'ETS (5' External Transcribed Spacer) are not affected but the levels of primary 45S and 35S transcripts are, respectively, increased and reduced. In contrast, precursors of 18S, 5.8S and 25S RNAs are rapidly undetectable upon heat stress. Remarkably, nucleolar structure, pre-rRNAs from major ITS1 processing pathway and ribosome profiles are restored after returning to optimal conditions, shedding light on the extreme plasticity of nucleolar functions in plant cells. Further genetic and molecular analysis to identify molecular clues implicated in these nucleolar responses indicate that cleavage rate at P site and nucleolin protein expression can act as a checkpoint control towards a productive pre-rRNA processing pathway

An In Vitro Approach To Study RNase III Activities of Plant RTL Proteins

International audienceRTL (RNase three-like) proteins belong to a distinct family of endonucleases that cleave double-stranded RNAs in plants. RTL1 to 3 are structurally related to the RNAse III from E. coli and formally belong to the class 1 of RNase III proteins. RTLs have conserved RNase III signature motif(s) and up to two dsRNA binding (DRB) domains. RTLs target and cleave coding and noncoding dsRNAs, including precursors of ribosomal (rRNA), small interference (siRNA), and micro (miRNA) RNAs. Interestingly, RTL proteins have stronger affinity than RNase III-Dicer proteins for dsRNA precursors of siRNAs, but not for miRNAs. However, very little is known of the structural and molecular bases directing and controlling RTL-RNA binding and activity. To address these questions, we have developed in vitro cleavage assays that combine recombinant RTL1 protein and in vitro transcribed or plant-extracted RNAs, RT-PCR, and primer extension experiments or analysis

Evaluating the magnitude and the stakes of peer effects analysing science and math achievement across OECD

What follows is an exercise aimed at estimating peer effects' impact on science and math test scores of secondary school students surveyed in 1995 by the International Education Agency across OECD countries. It is also to discuss their importance for educational policy, particularly regarding the highly sensitive issue of ability-grouping. Using this unique international database. This study assesses the magnitude of the peer effect relative to more traditional inputs. Referring to education policy stakes, we control for the presence of increasing or decreasing return. This study also checks for cross effects in order to determine whether peer effects matter more to low or high SES pupils, and whether their final impact on achievement is affected by the underlying level of heterogeneity within the group. Using a methodology, which a priori accounts for the clustering of the data within countries and schools/classrooms - i.e. fixed/random effect or hierarchical model - our analysis indicates that peer effects are strong determinants of both math and science achievement relative to individual SES and other school inputs. The presence of increasing of decreasing returns is not obvious. But we find systematic evidence that low-ability pupils are more sensitive to peer group characteristics. By contrast, this study also find that - for a given level of the peer effect - higher heterogeneity comes at a certain cost. In brief, these results provide no systematic evidence regarding grouping policies.

C1D family proteins in coordinating RNA processing, chromosome condensation and DNA damage response

Research on the involvement of C1D and its yeast homologues Rrp47 (S. cerevisiae) and Cti1 (S. pombe) in DNA damage repair and RNA processing has remained mutually exclusive, with most studies predominantly concentrating on Rrp47. This review will look to reconcile the functions of these proteins in their involvement with the RNA exosome, in the regulation of chromatin architecture, and in the repair of DNA double-strand breaks, focusing on non-homologous end joining and homologous recombination. We propose that C1D is situated in a central position to maintain genomic stability at highly transcribed gene loci by coordinating these processes through the timely recruitment of relevant regulatory factors. In the event that the damage is beyond repair, C1D induces apoptosis in a p53-dependent manner