原核與真核基因表達(dá)比較

1、Transcriptional RegulationThe regulation of transcription is one of the most important ways in which genes are regulated.Prokaryotes and Eukaryotes DiffernucleosomeProkaryotes and Eukaryotes DifferThere is a fundamentally different logic of gene regulation in bacteria and eukaryotes. In bacteria, th

2、ere is a active ground state and genes are often regulated by repressionProkaryotes and Eukaryotes DifferWhile eukaryotes have an inactive ground state and genes are typically regulated by activation.Allosteric RegulationSmall molecule effectors (效應(yīng)效應(yīng)物物)can alter the a c t i v i t y o f transcriptio

3、n factors.The lac OperonThe 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 b b-GalactosideaseLac Z

4、 gene product乳糖-半乳糖苷酶透性酶半乳糖苷轉(zhuǎn)乙酰酶異丙基-D-硫代半乳糖苷 甘油甘油 葡萄糖葡萄糖 Mutations in the lac OperatorlacOc 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 RepressorIn contrast, lacI mutants, which lack repressor act

5、ivity, can be complemented in merozygotes.Thus, lacI mutants are trans-acting.Mutations in the lac RepressorlacIs mutants, in which the repressor cannot bind to the inducer, are uninducible mutants.These lacI mutants are also trans-acting.The Core PromoterThe core promoter sequences we saw in our pr

6、evious discussion of prokaryotic transcription can be shown to have an impact on transcriptional activity.Glucose RepressionThe 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.Gl

7、ucose regulation is indirect, since it involves the second messenger cAMP (cyclic AMP). The cAMP signal tells the cell that glucose is absent.降解物激活蛋白降解物激活蛋白CAP Protein Bends DNAWhen the C A P - c A M P complex binds DNA it causes the DNA to bend.This bending appears to be the cause of the transcript

8、ional activation by the CAP-cAMP complex.The trp OperonThe trp operon is responsible for tryptophan biosynthesis.When would we expect to see trp operon activity?分支酸鄰胺苯甲酸 磷酸核糖 吲哚 The trp OperonThe trp operon is active when tryptophan is absent.Tryptophan acts as a small molecule co-repressor, binding

9、 the trp repressor and causing the trp repressor to bind DNA in the bound state.The trp OperonThe 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

10、 codons. If a ribosome stalls on these codons, transcription continues. Otherwise, it is terminated.Eukaryotic PromotersEukaryotic promoters are much more complex 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 CCA

11、AT and GC boxes are present in many promoters, but their presence is more variable.Eukaryotic PromotersThese boxes can also be defined mutationally. More recently, computation has been used to define them.EnhancersEnhancers are regulatory sites that can be quite far away in terms of the sequence.The

12、 EnhanceosomeReporter GenesA 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 TranscriptionThe compaction

13、of eukaryotic DNA into chromatin has a profound impact on transcriptional regulation.Dosage CompensationFemale mammals are mosaics, w i t h o n e o f t h e i r t w o X chromosomes inactivated.Inactivation involves the transcription of a non-coding gene called XIST.Other organisms have other means of

14、 dosage compensation, 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 d

15、iffer: 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.PWSASPWSMousemodelASMousemodelFrom A

16、nnu Rev Genomics & Hum GenetPrader-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

17、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.ImprintingImprinting is another type of epigenetic regulation(表觀遺傳調(diào)控), in which the parent that donates an allele determin

18、es whether it is active.The silenced allele has specific changes in DNA methylation that are established in the parents germline.Chromatin RemodelingYeast mutants that cannot change mating type or ferment sucrose蔗糖(swi and snf mutants) were identifiedMany factors specific to mating type switching or

19、 the regulation of invertase were identified, but one very general transcription factor (SWI-SNF) was identified.T h i s f a c t o r m o v e s nucleosomes away from the TATA box of genes.Perturbing Histone LevelsChanging the amounts of histone H4 can alter gene expression globally. If we complement 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 ModificationJust as DNA can be modified by methylation, histon