Sameet Mehta

Publications (8) View all

• Article: HDAC-mediated suppression of histone turnover promotes epigenetic stability of heterochromatin.

  Ozan Aygün, Sameet Mehta, Shiv I S Grewal
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  ABSTRACT: Heterochromatin causes epigenetic repression that can be transmitted through multiple cell divisions. However, the mechanisms underlying silencing and stability of heterochromatin are not fully understood. We show that heterochromatin differs from euchromatin in histone turnover and identify histone deacetylase (HDAC) Clr3 as a factor required for inhibiting histone turnover across heterochromatin domains in Schizosaccharomyces pombe. Loss of RNA-interference factors, Clr4 methyltransferase or HP1 proteins involved in HDAC localization causes increased histone turnover across pericentromeric domains. Clr3 also affects histone turnover at the silent mating-type region, where it can be recruited by alternative mechanisms acting in parallel to H3K9me-HP1. Notably, the JmjC-domain protein Epe1 promotes histone exchange, and loss of Epe1 suppresses both histone turnover and defects in heterochromatic silencing. Our results suggest that heterochromatic-silencing factors preclude histone turnover to promote silencing and inheritance of repressive chromatin.
  Nature Structural &#38 Molecular Biology 04/2013; · 12.71 Impact Factor
• Article: RNAi triggered by specialized machinery silences developmental genes and retrotransposons.

  Soichiro Yamanaka, Sameet Mehta, Francisca E Reyes-Turcu, Fanglei Zhuang, Ryan T Fuchs, Yikang Rong, Gregory B Robb, Shiv I S Grewal
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  ABSTRACT: RNA interference (RNAi) is a conserved mechanism in which small interfering RNAs (siRNAs) guide the degradation of cognate RNAs, but also promote heterochromatin assembly at repetitive DNA elements such as centromeric repeats. However, the full extent of RNAi functions and its endogenous targets have not been explored. Here we show that, in the fission yeast Schizosaccharomyces pombe, RNAi and heterochromatin factors cooperate to silence diverse loci, including sexual differentiation genes, genes encoding transmembrane proteins, and retrotransposons that are also targeted by the exosome RNA degradation machinery. In the absence of the exosome, transcripts are processed preferentially by the RNAi machinery, revealing siRNA clusters and a corresponding increase in heterochromatin modifications across large domains containing genes and retrotransposons. We show that the generation of siRNAs and heterochromatin assembly by RNAi is triggered by a mechanism involving the canonical poly(A) polymerase Pla1 and an associated RNA surveillance factor Red1, which also activate the exosome. Notably, siRNA production and heterochromatin modifications at these target loci are regulated by environmental growth conditions, and by developmental signals that induce gene expression during sexual differentiation. Our analyses uncover an interaction between RNAi and the exosome that is conserved in Drosophila, and show that differentiation signals modulate RNAi silencing to regulate developmental genes.
  Nature 11/2012; · 36.28 Impact Factor
• Source

  Available from: Sameet Mehta

  Article: A homolog of male sex-determining factor SRY cooperates with a transposon-derived CENP-B protein to control sex-specific directed recombination.

  Emiko Matsuda, Rie Sugioka-Sugiyama, Takeshi Mizuguchi, Sameet Mehta, Bowen Cui, Shiv I S Grewal
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  ABSTRACT: Schizosaccharomyces pombe cells switch mating type by replacing genetic information at the expressed mat1 locus with sequences copied from mat2-P or mat3-M silent donor loci. The choice of donor locus is dictated by cell type, such that mat2 is the preferred donor in M cells and mat3 is the preferred donor in P cells. Donor choice involves a recombination-promoting complex (RPC) containing Swi2 and Swi5. In P cells, the RPC localizes to a specific DNA element located adjacent to mat3, but in M cells it spreads across the silent mating-type region, including mat2-P. This differential distribution of the RPC regulates nonrandom choice of donors. However, cell-type-specific differences in RPC localization are not understood. Here we show that the mat1-M-encoded factor Mc, which shares structural and functional similarities with the male sex-determining factor SRY, is highly enriched at the swi2 and swi5 loci and promotes elevated levels of RPC components. Loss of Mc reduces Swi2 and Swi5 to levels comparable to those in P cells and disrupts RPC spreading across the mat2/3 region. Mc also localizes to loci expressed preferentially in M cells and to retrotransposon LTRs. We demonstrate that Mc localization at LTRs and at swi2 requires Abp1, a homolog of transposon-derived CENP-B protein and that loss of Abp1 impairs Swi2 protein expression and the donor choice mechanism. These results suggest that Mc modulates levels of recombination factors, which is important for mating-type donor selection and for the biased gene conversion observed during meiosis, where M cells serve as preferential donors of genetic information.
  Proceedings of the National Academy of Sciences 11/2011; 108(46):18754-9. · 9.68 Impact Factor
• Article: Over-represented promoter motifs in abiotic stress-induced DREB genes of rice and sorghum and their probable role in regulation of gene expression.

  Amrita Srivasta, Sameet Mehta, Angelica Lindlof, Sujata Bhargava
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  ABSTRACT: Genes coding for drought response element binding (DREB) proteins regulate transcription of a large number of downstream genes involved in the plant response to abiotic stresses. However the regulation of DREB genes themselves is not well understood. Using a bioinformatics approach, we identified the over-represented motifs in promoters of DREB genes of sorghum and rice as compared to all the other promoters in their genomes. Aligned orthologous promoter pairs of sorghum and rice DREBs were then used to identify co-localized motifs from among the over-represented ones, assuming that such motifs were likely to play a regulatory role. Finally the motifs over-represented in sorghum DREBs in comparison to their rice orthologs were identified. Results indicated over-representation of motifs pertaining to calcium, light, sugar, and hormone signaling in the DREB promoters. The co-localized motifs in DREB promoters were mainly those involved in abscisic acid-, light- and calcium-mediated regulation. These motifs along with others pertaining to ethylene signaling were over-represented in sorghum DREB promoters as compared to their orthologs from rice and could possibly contribute to its drought tolerance. Besides calcium, an integration of abscisic acid, ethylene, auxin and methyl jasmonate signaling was probably involved in regulating expression of the drought response through DREB transcription factors.
  Plant signaling & behavior 07/2010; 5(7):775-84.
• Source

  Available from: Jomon Joseph

  Article: Global regulator SATB1 recruits beta-catenin and regulates T(H)2 differentiation in Wnt-dependent manner.

  Dimple Notani, Kamalvishnu P Gottimukkala, Ranveer S Jayani, Amita S Limaye, Madhujit V Damle, Sameet Mehta, Prabhat Kumar Purbey, Jomon Joseph, Sanjeev Galande
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  ABSTRACT: In vertebrates, the conserved Wnt signalling cascade promotes the stabilization and nuclear accumulation of beta-catenin, which then associates with the lymphoid enhancer factor/T cell factor proteins (LEF/TCFs) to activate target genes. Wnt/beta -catenin signalling is essential for T cell development and differentiation. Here we show that special AT-rich binding protein 1 (SATB1), the T lineage-enriched chromatin organizer and global regulator, interacts with beta-catenin and recruits it to SATB1's genomic binding sites. Gene expression profiling revealed that the genes repressed by SATB1 are upregulated upon Wnt signalling. Competition between SATB1 and TCF affects the transcription of TCF-regulated genes upon beta-catenin signalling. GATA-3 is a T helper type 2 (T(H)2) specific transcription factor that regulates production of T(H)2 cytokines and functions as T(H)2 lineage determinant. SATB1 positively regulated GATA-3 and siRNA-mediated knockdown of SATB1 downregulated GATA-3 expression in differentiating human CD4(+) T cells, suggesting that SATB1 influences T(H)2 lineage commitment by reprogramming gene expression. In the presence of Dickkopf 1 (Dkk1), an inhibitor of Wnt signalling, GATA-3 is downregulated and the expression of signature T(H)2 cytokines such as IL-4, IL-10, and IL-13 is reduced, indicating that Wnt signalling is essential for T(H)2 differentiation. Knockdown of beta-catenin also produced similar results, confirming the role of Wnt/beta-catenin signalling in T(H)2 differentiation. Furthermore, chromatin immunoprecipitation analysis revealed that SATB1 recruits beta-catenin and p300 acetyltransferase on GATA-3 promoter in differentiating T(H)2 cells in a Wnt-dependent manner. SATB1 coordinates T(H)2 lineage commitment by reprogramming gene expression. The SATB1:beta-catenin complex activates a number of SATB1 regulated genes, and hence this study has potential to find novel Wnt responsive genes. These results demonstrate that SATB1 orchestrates T(H)2 lineage commitment by mediating Wnt/beta-catenin signalling. This report identifies a new global transcription factor involved in beta-catenin signalling that may play a major role in dictating the functional outcomes of this signalling pathway during development, differentiation, and tumorigenesis.
  PLoS Biology 01/2010; 8(1):e1000296. · 11.45 Impact Factor