Alterazione della cromatina  Modificazione degli istoni  Rimodellamento della cromatina.

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Watson et al. , BIOLOGIA MOLECOLARE DEL GENE, Zanichelli editore S. p

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Alterazione della cromatina  Modificazione degli istoni  Rimodellamento della cromatina

Watson et al., BIOLOGIA MOLECOLARE DEL GENE, Zanichelli editore S.p.A. Copyright © 2005

Biologia molecolare - Robert F. Weaver Copyright © 2005 – The McGraw-Hill Companies srl

Chromatin modification  Histone acetylation is associated with activation of gene expression.  Deacetylated chromatin may have a more condensed structure.  Transcription activators are associated with histone acetylase activities in large complexes.  Transcription repressors are associated with histone deacetylase activities.  Histone acetylases vary in their target specificity and we do not yet know whether acetylation affects transcription in a quantitative or qualitative way.

Watson et al., BIOLOGIA MOLECOLARE DEL GENE, Zanichelli editore S.p.A. Copyright © 2005

Attività di rimodellamento della cromatina (in vitro)  “Scivolamento” dei nucleosomi  Trasferimento dell’ottamero istonico ad un’altra molecola di DNA  Formazione di dinucleosomi  Alterazione della struttura nucleosomale

Watson et al., BIOLOGIA MOLECOLARE DEL GENE, Zanichelli editore S.p.A. Copyright © 2005

Tipi di complessi di rimodellamento della cromatina  SWI/SNF: Swi/Snf di lievito RSC di lievito  ISWI:NURF di Drosofila CHRAC di Drosofila ACF di Drosofila  CHD (Mi-2/NuRD)  INO80

Watson et al., BIOLOGIA MOLECOLARE DEL GENE, Zanichelli editore S.p.A. Copyright © 2005

Chromatin remodeling  Chromatin structure is stable and cannot be changed by altering the equilibrium of transcription factors and histones.  There are several chromatin remodeling complexes that use energy provided by hydrolysis of ATP.  The SWI/SNF, RSC, and NURF complexes all are very large; there are some common subunits.  A remodeling complex does not itself have specificity for any particular target site, but must be recruited by a component of the transcription apparatus.

Polycomb and Trithorax  Polycomb group proteins (Pc-G) perpetuate a state of repression through cell divisions.  The PRE is a DNA sequence that is required for the action of Pc-G.  The PRE provides a nucleation center from which Pc-G proteins propagate to maintain an inactive structure.  No individual Pc-G protein has yet been found that can bind the PRE.  Trithorax group proteins antagonize the actions of the Pc-G.