Outline Chromatin modification Capping Polyadenylation Welch – 3:15 pm Thursday Oct 30.

Figure 13.3c

Figure 13.4b

Figure 13.4c

Two Complexes Play Major Roles HDAC - Histone Deacetylases HAT - Histone Acetyl Transferase

Histone acetylation amino groups of lysine side chains unacetylated histones tend to repress transcription acetylated histones tend to activate transcription Histone acetyl transferase (HAT) Histone deacetylase

Acetylation continued Acetylation of histone tails neutralizes some of the positive charge, causing them to relax their grip on the DNA. Reduces nucleosome cross-linking. That is; the interaction between histones in neighboring nucleosome. (eg. H4 in one nucleosome and H2A- H2B dimer in the next one.

Acetylation continued Also some TFs recognize acetylated histones. eg. TAFII250 has a double bromodomain and recognizes low level acetylated histones. Once bound it is a HAT and increases acetylation.

Figure 13.29

Proposed Mechanism of Histone Deacetylation and Hyperacetylation in Yeast Transcription

Published by AAAS T. Jenuwein et al., Science 293, (2001) Models for euchromatic or heterochromatic histone tail modifications

Models for euchromatic or heterochromatic histone tail modifications

Published by AAAS T. Jenuwein et al., Science 293, (2001) Translating the "histone code."

Figure HAT Chromatin remodeling By SWI/SNF complex

Summary Chromatin is a dynamic structure Histone tails are posttranslationally modified Different types of modifications constitute “Histone Code” Different types of modifications are recognized by different proteins to translate the “Histone Code?”.