Chapter 18 Control of Gene Expression in Eukaryotes 1 mechanisms of gene regulation in eukaryotes a DNA wrapped around proteins to create DNA Protein complex called chromatin b Before transcription can begin in eukayotes the stretch of DNA w promoter must be released from tight interactions w proteins so that RNA Polymerase can contact promoter c So chromatin remodeling must occur prior to transcription 2 Second Level of regulation a RNA Processing steps required to produce mature mRNA from primary RNA transcript b often alternative splicing patterns occur diff combinations of exons unite and make diff gene products as result 3 Third Level of Regulation a mRNA life span is regulated in eukaryotes b mRNAs that are active for a long time are translated more than those w short life span c All 6 control points are used at certain times in eukaryotic cell 4 Chromatin Remodeling a For arrival of signaling molecule to result in transcription of gene chromatin around target gene must be remodeled b DNA is packed inside nucleus so tightly RNA polymerase cant access it some of this packing is done by supercoiling 5 Chromatin s Basic Structure draw a most abundant DNA associated proteins belong to a group called the histones b Chromatin consists of DNA complexed w histones and proteins c Chromatin looked like beads on a string the beads are called nucleosomes d X ray crystallographic data indicated that each nucleosome consists of DNA wrapped twice around a core of 8 histone proteins histone H1 seals DNA to each set of 8 nucleosomal histones e Intimate associaton bet DNA and histones occurs in part bec DNA is neg charged and histones are pos charged f H1 histones interact w each other and histones in other nucleosomes to produce the 30 nanometer fiber g When chromosomes condense prior to mitosis or meiosis the scaffold proteins and 30nm fibers are folded and packed into more compact structure h chromatin structure has profound implications for control of gene expression 6 Chromatin Structure is Altered in Active Genes a the close interaction bet DNA and histones must be altered for RNA polykmerase to make contact w DNA b Chromatin must be relaxed or decondensed for RNA polymerase to bind to promoter 7 Closed DNA Protected from DNAse a DNase is an enzyme that cuts DNA at random locations works efficiently only if DNA is in open configuration b Weintraub and Groudine compared chromatin structure in genes of blood cells B globin and ovalmine genes c B globin is protein part of hemoglobin in red blood cells Ovalbumn is major protein of egg white d After treating blood cells w DNase then analyzing state of the B globin and obalbumin genes they found the DNase cut up the B globin more than the ovalbumin gene e So chromatin in blood cells was in open configuration at B globin gene but closed at ovalbumin gene 8 Histone Mutants another type of evidence in support of chromatin remodeling hypothesis a In mutant yeast cells that dont produce usual complement of histones genes not normally transcribed are transcribed at high levels b the lack of histone proteins prevented assembly of normal chromatin c Absence of normal histone DNA interactions promotes transcription so presence of it must prevent transcription d in normal default state eukaryotic genes are off This is all a new mechanism of negative control 9 How is Chromatin Altered a Chromatin remodeling multi protein machines which reshape chromatin through series of ATP dependent reactions b remodeling complexes contact DNA on surface of nucleosome twist it in way that allows DNA to loop out from protein core where it can be transcribed c Other players in chromatin remodeling add small molcules to histone proteins 8 types of chemical modifications have been discovered two of best studied processes are acetylation and methylation d acetylation associated w positive control activation of genes through binding of regulatory protein methylation correlated w activation of inactivation depending on which histones are altered 10 Histone Transferases HATs a enzymes that modify chromatin acetylate positively charged lysine residues in histones b When a HAT adds acetyle group to selected histones marked proteins act as binding sites for chromatin remodeling complexes that open DNA c chromatin is recondensed by histone deacetylases HDACs remove acetyle groups added by HATs d If HATs are on on switch for transcription HDACs are off State of histone proteins complexed w DNA is critical determinant of whether transcription occurs 11 Chromatin Modification Can Be Inherited Histone Code Hypothesis a Histone code precise patters of chem modifications of histones contain information analogous to way the genetic code stores information the histone code influences whether or a not a particular gene is expressed b most or some of chemical modifications that distinguish a brain from muscle associated cell are passed to daughter cell at mitosis c Histone modifications are example of epigenetic inheritance a k a patters of inheritance not due to diff in DNA seq d Muscle and brain cells differ in part bec of differently modified histones not diff types of genes 12 Regulatory Sequences and Regulatory Proteins a Once promoter has been exposed to chromatin remodeling the first step in transcription is interaction w the TATA binding protein b Other proteins and DNA seq are involved in controlling gene expression 13 Regulatory Sequences that Are Near the Promoter a Regulatory sequences sections of DNA that are involved in controlling the activity of genes b When galactose is absent S cerevisiae cells produce tiny quantities of enzyme req to metabolize it when galactose is present transcription of genes encoding the enzyme increases c Result discovery of mutant cells that dont produce any of the 5 enzymes req for galactose metabolism even if galactose was present SO 1 the 5 genes are regulated together 2 Normal cells have a CAP like regulatory protein that exerts positive control over the 5 genes 3 the mutant cells have loss of function mutation that disables the regulatory protien 14 Promoter proximal elements a regulatory sequences located close to promoter and bind regulatory proteins b Have seq unique to specific genes 15 Reg Sequences Far From Promoter a Antibodies proteins that bind to specific sites of other molecules bind to viruses and bacteria and mark them for destruction b Tonegawa used techniques to place copies of an intron in new locations when placed intron
View Full Document
Unlocking...