Unit 3 Regulation of Gene Expression
translation of info encoded in a gene into a protein or RNA
Advantages of Gene Regulation
*Cellular control of structure & fx, *versatility and adaptation, *conservation of energy, *development- stages of protein synthesis
Regulated Transcription in Prokaryotes
Mostly regulated on the level of *initiation; one promoter for all genes in an operon
prevent binding of RNA pol to promoter. *Constitutively active, *Corepressor required to activate repressor (incr. [Trp] inhibits transcription of Trp operon. Trp is corepressor of Trp operon)
stimulate transcription, usually nutrients or their metabolites, bind represser and release it from operator. No inducer = no expression.
encodes for lactose metabolism; allolactose (lactose metabolite) is an inducer. Glucose prevents activation of lac operon. Enzymes for Glucose met constantly produced.
cAMP-CRP Lac Operon Cont.
necessary coactivator (low transcription w/o). Low Glucose leads to incr. [cAMP], cAMP binds CRP and binds to operon and stim. pol binding and transcription
Attenuation of Transcription
Trp operon: [Trp] is high, ribosome rapidly translates generating a hairpin loop between sequences 3 & 4 = termination. [Trp] is low, ribosome stall at Trp codons and hairpin does not form & entire operon is transcribed.
Regulation of Gene Expression in Eukaryotes
Chromatin structure: chromatin remodeling and gene rearrangement, Transcription: TFs affect binding of RNA pol, Maturation of mRNA- processing incl. alternative splicing, Translation: initiation and mRNA stability
What happens if a promoter region is a part of nucleosome?
Transcription does not occur- requires chromatin remodeling
some genes are only actively transcribed on paternally or maternally inherited chromosomes.
Gene expression is also affected by rearrangements, amplification, and deletion
Epigenetic Changes in Chromatin
DNA methylation and other histone modifications alter structure of chromatin and gene expression- tissue and cell specific
intellectual and developmental delay; inability to express gene UBE3A on paternally inherited *chromosome 15 by imprinting; mutation (15%) deletion (70%) of maternal gene- paternal imprinted gene cannot be expressed. Affects 1:12K-20K
Prader-Willi Syndrome (PWS)
Loss of genes active only on paternal *chromosome 15 (opposite angelman syndrome); genes for small nucleolar RNAs (snoRNAs); low muscle tone & instable appetite in childhood- hyperphagia & obesity; 70% deletion of region of paternal chromosome 15- maternal uniparental disomy (UPD)
Prader-Willi Syndrome (PWS) Cont.
25% caused by two copies of maternal chromosome 15. Affects 1:10K-30K
dwarfism caused by imprinting error; loss of methylation of H19 & IGF2 genes on *chromosome 11 leads to slow growth, *can receive 2 maternal chromosome 7- no expression of genes only in paternal copy. Incidence 1:3K-100K
DNA Methylation and Cancer
DNA *hypermethylation can *silence tumor suppressors (DNA repair, neg regulators of cell cycle, neg regulators of DNA repl, & chromosomal stability) and *hypomethylation activate proto-oncogenes (cMYC & H-RAS)
Methylation of DNA
Cytosine can be methylated by *DNA methyltransferases (~1% of bases are methylated); genes are less actively transcribed (globin genes methylated in cells where not expressed); regulated gene expression during differentiation. *Overall hypomethylation in cancer cells
ATP-dependent Chromatin Remodeling Complexes
Unwind DNA from Nucleosome
Covalent modification of Histones
Histone Acetyltransferases (HAT) create active chromatin.
Histone deacetylases (HDAC) create repressed chromatin
*Lys residues in tail of histone are affected
Basal Transcription Complex
TBP and general TFs complexed with RNA pol II
Gene Control Regions
TFs can bind and increase transcription 1K-fold; located upstream and downstream of promoter
Gene Specific Transcription Factors
Activators, Inducers, Repressors, Nuclear Receptors; *have a DNA binding domain, domain for mediator proteins (bind coactivators, corepressors, or TBP-associated factors that can be gene-specific or general); steroid hormone bind corepressor or coactivators; some TFs can induce or inhib.
Regulate transcription through nuclear receptors (NRs)- gene specific TFs
Hormone Response Elements (HRE)
Nuclear receptors (NRs) that bind regulatory sequence and induce or repress transcription
Nuclear Receptors (NRs) Domains
Several Domains: ligand binding domain, DNA binding domain, dimerization domain; transactivation domain binds coactivator proteins; nuclear localization signal
bind cortisol @ cytosol (Steroid-Thyroid Hrmn Recptr), recptr dissociates from heat shock protein, expose nuclear localization signal (NLS), form dimer, translocate to nucleus, bind glucocorticoid response element (GRE), transactivation domain bind mediator proteins and activate transcription.
Thyroid & Retinoid X Receptors
(Steroid-Thyroid Hormone Receptor); bound to DNA constitutively, Thyroid & Retinoid form heterodimer; absence of thyroid hormone dimer binds corepressor and inhibits transcription. Thyroid hormone causes binding of coactivator and activation. No hrmn receptor = disease
Androgen Insensitivity Syndrome (AIS)
patients produce androgens but lack receptor; complete (ACIS)- woman with nearly normal female body despite XY karyotype; ACIS affects 2-5:100K; Incomplete can include other disorders: *Gynecomastia (breast development in men), *Cryptorchidism (1 or both testes fail to descend after birth)
TFs Activation (Regulation)
availability of coactivators and mediators are critical for TF activation;
TFs Activity Modulation (Regulation)
activity can be modulated by changes in [TF], binding of inhibitor or stimulator, stim. of nuclear entry, phosphorylation (CREB by PKA or MAP kinase)
Alternative Splicing of mRNA
some genes can be spliced in two or more alternative ways (~80% of genes), produces diverse set of proteins, alternative types of splicing ie exon skipping
alternative splicing results in apoB48, which increases fat absorption in the intestines, or apoB100, which is associated with more atherogenic lipoproteins. Low or no hepatic apoB mRNA leads to incr. levels of VLDL & LDL
Degradation of Eukaryotic mRNA
progressive shortening of poly(A) tail; loss of tail leads to rapid degradation (mRNA turnover) by *cytoplasmic exosome @ 3', or in *P body (yeast) or *cytoplasmic foci (humans) at 5' end decapped mRNA
main iron storage protein in cel; synth when iron is high; iron respone element (IRE) near 5' end of mRNA. Low iron- IRE-BP prevents translation. High iron- IRE-BP binds iron, dissociation from IRE, translation of ferritin mRNA
Iron transporter in blood. Low iron- more receptor & vice versa through same function of ferritin except mRNA is degraded w/ high iron for less receptor.
Prokaryote vs Eukaryote Regulation of Gene Expression: Prokaryote
Prokaryotes: inductinon and repression of transcription by repressors, inducers, corepressors, and coactivators
Prokaryote vs Eukaryote Regulation of Gene Expression: Eukaryote
Eukaryotes: chromatin modification and remodeling- HATs & HDACs, DNA & histone methyltransferases & demethylases, regulation of transcription initiation- + & - TFs, translation- mRNA editing, mRNA degradation, binding of ribosome to mRNA