1、Regulation of Gene ExpressionSection 1Principles and Concepts 1.1 ConceptsGene:A DNA segment that contains the all genetic information required to encodes RNA and protein molecules.Genome:A complete set of genes of a given species.Gene expression:A process of gene transcription and translation.Speci
2、ficity of gene expression Temporal specificity(also called stage specificity):why in the infant not in the aged ones?Spatial specificity(also called tissue specificity):why in liver not in brain?Specificity of gene expression Type of gene expressiona.Constitutive expressionSome genes are essential a
3、nd necessary for life,and therefore are continuously expressed,such as those enzymes involved in TAC.These genes are called housekeeping genes.b.Induction and repression The expression levels of some genes fluctuate in response to the external signals.Some genes demonstrate higher expression level o
4、nce being activated.It is called induced expression.On the other hand,some genes are repressed and their expression levels are lower.It is called repressed expression.1.2 Regulatory Elements Gene expression is a multiple-level process.Transcription initiation is a key point of controlling gene expre
5、ssion.Basic elements that regulate the transcription include:a.Special DNA sequencesb.Regulatory proteinsc.DNA-protein interaction and protein-protein interactiond.RNA polymerase For prokaryotic systems:Operon is composed of structural genes,promoter,operator,and other regulatory sequences.a.Special
6、 DNA sequenceOther requlatory sequenceOperatorPromoter Sturctural genes The DNA sequence that RNA-pol can bind to and initiate the transcription.Promoterpromoteroperatorstructural generepressorRNA polThe DNA sequence adjacent to the structural genes that the repressor protein can bind to and prevent
7、 the transcription of structural genes.Operator structural genestartABCis-acting elements is the special DNA sequence that can affect the expression of its own gene.For eukaryotic systems:b.Regulatory proteinsFor prokaryotic systems:Specific factor:It facilitates the binding of RNA-pol to particular
8、 DNA sequence.Repressor:It binds to the operator and prevent the transcription,known as negative regulation.Activator:It associates with DNA near the initiation point,resulting in the increase of RNA-pol binding affinity and the enhancement of the transcription efficiency.For eukaryotic systems:The
9、regulatory proteins are called transcription factors(TF).After expression,TF will interact with the cis-acting elements to activate another genes.Therefore,they are referred to as trans-acting factors.Trans-acting factorsaADNAmRNAprotein AbA The regulation is implemented through numerous interaction
10、s between cis-acting elements and trans-acting factors.They are non-covalent bond.c.DNA-protein interactions Proteins may have to interact with each other prior to the DNA binding.Proteins can form a homo or hetero-dimer form to function properly.Present in prokaryotes as well as eukaryotes.Protein-
11、protein interactionsSection 2Gene Regulation of Prokaryotic Systems Common features Prokaryotic genes are polycistron systems,that is,several relevant genes are organized together to form a transcription unit-operon.The majority of gene regulation is negative.Inducers are used to remove the repressi
12、on.Operon is a coordinate unit for the regulation.Transcription initiation is the key point for regulation.Translation can also be regulated.2.1 Regulation of Transcription AYZOPIstructural genepermease-galactosidaseregulatory siteoperatorpromoterCAP-binding siteregulatory genetransacetylaseStructur
13、e of lac operonMetabolism of lactose Bacteria do not express these three enzymes when glucose is available.However,bacteria produce those enzymes if lactose is present and glucose is absent.Inducible expression Sequence of lac operon lac operon(TTTACA/TATGTT)is a weak promoter,and has a basal expres
14、sion level.CAP(Catabolite gene activator protein)binding site is at-60 region.CAP is a homodimer with binding ability to DNA and cAMP.Glucose inhibits the formation of cAMP.When glucose is present,cAMP is lower.Only after glucose is exhausted,cAMP becomes higher.The CAP-cAMP complex is formed,and th
15、is complex binds to the CAP binding site on lac operon.AYZOPImRNARNA polWhen lactose is absent,no lac gene is expressed.Situation 1 lacI gene has its own promoter,and its expression can produce LacI repressor.The tetrameric Lac repressor binds to the lac operator site Olac.The binding blocks the RNA
16、-pol moving on DNA template,and no lacZ,lacY,and lacA are expressed.AYZOPIrepressormRNAlactoseallolactoseRNA polgalactosidaseSituation 2When lactose is present,lacZ,lacY,and lacA genes are expressed.The galactosidase is weakly expressed(at the basal level).When lactose is present,it is converted to
17、allolactose or galactose that binds to the repressor.The repressor can no longer bind to the operator,and lac gene can be expressed.Galactose,Allolastose and IPTG are referred to as inducer.Inducers The lacZYA RNA transcript is very unstable and could be degraded quickly.Therefore,the synthesis of t
18、hree enzymes will be cease under normal condition.Presence of lactose AYZOPIRNA polCAPWhen glucose is present,the cAMP is low,no CAP-cAMP is formed and the expression of the lac operon is still low.Situation 3 AYZOPICAPcAMPRNA polWhen glucose is absent and lactose is present,the CAP-cAMP complex bin
19、ds to the CAP site to activate the lac gene.Situation 4 OOOORNA pollactoseno lactoseRNA polRNA polRNA pol Coordinate expressionNo glucoseGlucose 2.2 Transcription Attenuation The trp operon is one of the constitutive genes expressed at the basal level.The structural gene of trp operon encodes 5 enzy
20、mes used for the synthesis of Trp.Trp operon The trp repressor gene can be expressed,but it does not bind to the operator.When Trp is more than enough,the repressor will form a complex with Trp.The complex binds to the operator,blocking the synthesis of Trp.ABCDEOPRregulatory regionsstructuralgeneTr
21、pTrp mRNAattenuated mRNAtrp LattenuatorTrp operon Attenuation mechanism In addition to the repressor regulation,trp gene has a fine tuning mechanism called attenuation.The trp operon is regulated using attenuation mechanism at the translation level.Leader sequence Possible hairpins1/2 and 3/4 hairpi
22、n structure2/3 hairpinstructure High Trp concentration Low Trp concentrationSection 3Regulation of Eukaryotic Transcription Structural featuresLarge genome:3 x 109 bps,35 000 genesMonocistronRepeated sequences:different lengths and different frequencies.Often inverted repeatsSplite genes:separated b
23、y introns and exons alternatively Regulation features1.RNA-pol:3 forms(I,II,and III)for different RNAs2.Changes of chromosomal structure Hypersensitive site Base modification Isomer-conversion Histone changes 3.Positive regulation4.Transcription and translation are separated5.Post-transcriptional mo
24、dification6.Regulation through intercellular and intracellular signals 3.1 Cis-acting elements They are specific DNA sequences,each of which regulates transcription of one or more genes.They usually have consensus sequences.Promoter:TATA box,CAAT box,and GC box,Sequence:TATAAAA Location:-25 -30 bp F
25、unction:It is the binding site for TFII D,which is required for RNA polymerase binding.It controls the veracity and frequency of transcriptional initiation.TATA box Sequence:GCCAAT Location:-70 bp Function:It is the binding site for CTF1(CAAT-binding transcription factor)and C/EBP(enhancer binding p
26、rotein).CAAT box Sequence:GGGCGGLocation:-30 -110 bpFunction:It is the binding site for a protein called Sp1.GC box It is a DNA sequence that can determine the temporal and spatial specificities of expression and increase the promoter activity.enhancer It is a negative regulation element.It will rep
27、ress the transcription once interacted with specific proteins.Silencer 3.2.Trans-acting factors They are the proteins that bind indirectly to cis-acting elements and then regulate the transcription initiation.The trans-acting factors can be transcription factors(TF).transcription factors General tra
28、nscription factors Special transcription factors Transcription activators EBP(enhancer binding protein)Transcription inhibitors General structure of TF DNA-binding domain Activation domain Protein-protein interaction domain Promoter and regulatory proteins General structure of TF CTD of RNA-pol II i
29、s an important point of interaction with mediators and other protein complexes.Cofactors facilitate the TF assembly.Transcription repressor3.3 DNA-protein interactions Regulatory proteins have discrete DNA-binding domains of particular structure,i.e.,binding motif.The AA side chains of regulatory proteins interact with bases of DNA through H bonds.Yeast activator protein GCN4Leucine zipper Zinc fingerSteroid hormone receptor Mouse regulatory protein Zif268Zinc finger Helix-loop-helix Helix-loop-helixHuman transcription factor MAX Helix-turn-helixLac repressor Helix-turn-helixTrp repressor
