原核与真核基因表达比较.ppt

Transcriptional Regulation,The regulation of transcription is one of the most important ways in which genes are regulated.,Prokaryotes and Eukaryotes Differ,nucleosome,Prokaryotes and Eukaryotes Differ,There is a fundamentally different logic of gene regulation in bacteria and eukaryotes.In bacteria,there is a active ground state and genes are often regulated by repression,Prokaryotes and Eukaryotes Differ,While eukaryotes have an inactive ground state and genes are typically regulated by activation.,Allosteric Regulation,Small molecule effectors(效应物)can alter the activity of transcription factors.,The lac Operon,The lac operon was one of the first genetic regulatory systems to be understood in detail.The lac operon is responsible for lactose utilization,and it is only active when lactose(or related galactosides)are present.,Lactose and b-Galactosidease,半乳糖,葡萄糖,乳糖,-半乳糖苷酶,透性酶,半乳糖苷转乙酰酶,异丙基-D-硫代半乳糖苷,甘油,葡萄糖,Mutations in the lac Operator,lacOc mutants have an altered sequence that does not allow the repressor to bind.This results in a cis-acting constitutive phenotype.,Mutations in the lac Repressor,In contrast,lacI mutants,which lack repressor activity,can be plemented in merozygotes.Thus,lacI mutants are trans-acting.,Mutations in the lac Repressor,lacIs mutants,in which the repressor cannot bind to the inducer,are uninducible mutants.These lacI mutants are also trans-acting.,The Core Promoter,The core promoter sequences we saw in our previous discussion of prokaryotic transcription can be shown to have an impact on transcriptional activity.,Glucose Repression,The lac operon is also regulated by glucose repression-it is only active when lactose is present and glucose is absent.This is because glucose is a preferred carbon source.Glucose regulation is indirect,since it involves the second messenger cAMP(cyclic AMP).The cAMP signal tells the cell that glucose is absent.,降解物激活蛋白,CAP Protein Bends DNA,When the CAP-cAMP plex binds DNA it causes the DNA to bend.This bending appears to be the cause of the transcriptional activation by the CAP-cAMP plex.,The trp Operon,The trp operon is responsible for tryptophan biosynthesis.When would we expect to see trp operon activity?,分支酸,邻胺苯甲酸,磷酸核糖,吲哚,The trp Operon,The trp operon is active when tryptophan is absent.Tryptophan acts as a small molecule co-repressor,binding the trp repressor and causing the trp repressor to bind DNA in the bound state.,The trp Operon,The trp operon is also regulated by attenuation.Attenuation is an interaction between transcription and translation.There is a short open reading frame(ORF)upstream of the trp genes.This ORF has two Trp codons.If a ribosome stalls on these codons,transcription continues.Otherwise,it is terminated.,Eukaryotic Promoters,Eukaryotic promoters are much more plex and the elements present are much more variable.The TATA box is one of the most important,and the majority of promoters have it.The CCAAT and GC boxes are present in many promoters,but their presence is more variable.,Eukaryotic Promoters,These boxes can also be defined mutationally.More recently,putation has been used to define them.,Enhancers,Enhancers are regulatory sites that can be quite far away in terms of the sequence.,The Enhanceosome,Reporter Genes,A reporter gene like-galactosidase can be used to study different enhancer elements.Here,Drosophila embryos stained for-gal activity are shown regulated by dpp enhancer elements are shown.,Lateral Ectoderm(LE),Imaginal Disc(ID),Chromatin and Transcription,The paction of eukaryotic DNA into chromatin has a profound impact on transcriptional regulation.,Dosage Compensation,Female mammals are mosaics,with one of their two X chromosomes inactivated.Inactivation involves the transcription of a non-coding gene called XIST.Other organisms have other means of dosage pensation,such as hypertranscription of the male X-linked genes(Drosophila)or reducing X-linked transcription(C.elegans hermaphrodites).,Prader-Willi&Angelman Syndromes,Both of these genetic disorders are caused by deletion of a segment of chromosome 15.However,the syndromes differ:Prader-Willi Syndrome-obesity,mental retardation,short stature.Angelman Syndrome-uncontrollable laughter,jerky抽筋的 movements,and other motor and mental symptoms.The syndrome that develops depends upon the parent that provided the mutant chromosome.,PWS,AS,PWSMousemodel,ASMousemodel,From Annu Rev Genomics&Hum Genet,Prader-Willi&Angelman Syndromes,Prader-Willi Syndrome-develops when the abnormal copy of chromosome 15 is inherited from the father.Angelman Syndrome-develops when the abnormal copy of chromosome 15 is inherited from the mother.The differences reflect the fact that some loci are imprinted-so only the allele inherited from one parent is expressed.The chromosome 15 region contains genes with both maternal and paternal imprinting.,Imprinting,Imprinting is another type of epigenetic regulation(表观遗传调控),in which the parent that donates an allele determines whether it is active.The silenced allele has specific changes in DNA methylation that are established in the parents germline.,Chromatin Remodeling,Yeast mutants that cannot change mating type or ferment sucrose蔗糖(swi and snf mutants)were identifiedMany factors specific to mating type switching or the regulation of invertase were identified,but one very general transcription factor(SWI-SNF)was identified.This factor moves nucleosomes away from the TATA box of genes.,Perturbing Histone Levels,Changing the amounts of histone H4 can alter gene expression globally.If we plement a histone H4 mutant with a plasmid that has H4 under GAL gene control it is possible to deplete histones by shifting the cells to glucose medium.Histone depletion activates genes that are inducible.,Histone Modification,Just as DNA can be modified by methylation,histones can be modified by methylation and acetylation.Acetylation is better studied,and it is correlated with gene activation.Some transcription factors,like GCN5,are histone acetyltransferases.Repressors,like RPD3,are histone deacetylases,The Enhanceosome,The enhanceosome will act to recruit histone acetyltransferase activity,which results in the recruiting SWI-SNF(moving nucleosomes)and ultimately TATA-binding protein(TBP)and RNA pol II.,The Histone Code,The histone code can be“inherited”by modifying newly synthesized nucleosomes after DNA replication.This represents an example of epigenetic inheritance.,