Biochemistry, 4th Edition P98. Continuing Garrett and Grisham's innovative conceptual and organizing framework, "Essential Questions," BIOCHEMISTRY guides students through course concepts in a way that reveals the beauty and usefulness of biochemistry in the everyday world. Streamlined for increased clarity and readability, this edition also includes new photos and illustrations that show the subject matter consistently throughout the text. New end-of-chapter problems, MCAT practice questions, and the unparalleled text/media integration with the power of CengageNOW round out this exceptional package, giving you the tools you need to both master course concepts and develop critical problem-solving skills you can draw upon. | How Are Genes Transcribed in Eukaryotes 933 Covalent Modification of Histones Chromatin is also remodeled through the action of enzymes that covalently modify side chains on histones within the core octamer. These modifications either diminish DNA histone associations through disruption of electrostatic interactions or introduce substitutions that can recruit binding of new protein participants through protein-protein interactions. Initial events in transcriptional activation include acetyl-CoA-dependent acetylation of e-amino groups on lysine residues in histone tails by histone acetyltransferases HATs Figure . The histone transacetylases responsible are essential components of several megadalton-size complexes known to be required for transcription coactivation co-activation in the sense that they are required along with RNA polymerase II and other components of the transcriptional apparatus . Examples of such complexes include the TFIID some of whose TAFns have HAT activity the SAGA complex which also contains TAFIIs and the ADA complex. A-Acetylation suppresses the positive charge in histone tails diminishing their interaction with the negatively charged DNA. Phosphorylation of Ser residues and methylation of Lys residues in histone tails also contribute to transcription regulation Figure . Attachment of small proteins to histone C-terminal lysine residues through ubiquitination and sumoylation see Chapter 31 are two additional forms of covalent modification found in nucleosomes. Collectively these modifications create binding sites for proteins that modulate chromatin structure such as the chromatin-remodeling complexes with bromodomains that interact specifically with acetylated lysine residues and chromodomains that bind to methylated lysine residues. A histone code has emerged. Covalent Modification of Histones Forms the Basis of the Histone Code A code based on histone-tail covalent modifications determines gene expression through .
