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Chapter 7 Operons: Fine Control of Bacterial Transcription

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1 Chapter 7 Operons: Fine Control of Bacterial Transcription
Student learning outcomes: Explain basic features of regulation of operons: classic catabolic lac and ara; anabolic system trp. role of repressors, operators, positive controls Describe briefly role or riboswitches in regulation Lac repressor (pink) and CAP-cAMP (blue) binding lac operator/promoter region

2 Bacterial transcription
Regulation of gene expression at transcriptional level is very important for prokaryotes: - conserves energy, permits rapid response - mRNA in prokaryotes is short-lived - regulation involves protein-ligand and protein-DNA interactions primarily Important Figs: 1, 2, 3*, 5*, 6, 7, 10, 11, 12, 13, 14, 15*, 16, 18, 19, 20, 21*, 26, 27*, 28, 29, 31, 32, 34 Review Q: 2, 3, 4, 5, 10, 11, 13, 16, 19, 21 Analyt Q: 1, 2, 3, 4

3 7.1 lac Operon of E. coli: catabolism of lactose
First operon discovered (Jacob & Monod 1940s-1950s) 3 genes coding for proteins to use of sugar lactose Galactoside permease (lacY) transports lactose into cells b-galactosidase (lacZ) cleaves lactose into galactose and glucose Galactoside transacetylase (lacA) function unclear Fig. 1

4 Genes of the lac Operon Genes adjacent on chromosome
lacZ = b-galactosidase lacY = galactoside permease lacA = galactoside transacetylase 3 genes transcribed as 1 mRNA, polycistronic message that starts from one promoter Each cistron, or gene, has own ribosome binding site Each cistron can be translated by separate ribosomes

5 Control of lac Operon Glucose preferred substrate for E. coli; only make lac enzymes if need to use lactose as carbon source lac operon is tightly controlled, 2 types of control: Negative control for catabolic systems: ‘transcription is off unless turn on’; inducer must remove repressor from operator Activator, positive factor CAP protein: responds to low glucose (energy) by stimulating transcription of lac operon

6 Negative Control of the lac Operon
‘Off until needed to turn on’ Off-regulation by lac repressor: Product of lacI gene Tetramer of 4 identical polypeptides Binds operator just upstream of promoter When repressor binds operator, operon is repressed Repressor bound to operator prevents RNA polymerase from initiating at promoter lac operon is repressed as long as no lactose is available or needed (i.e. glucose available)

7 Negative Control of the lac Operon
Fig. 7.3

8 Inducer of the lac Operon
Repressor is an allosteric protein Binding of molecule (inducer) to one place on protein changes shape of a different site on the protein Alters its interaction with a second molecule (DNA) Inducer binds each monomer of repressor Repressor changes conformation to favor release from operator (the other molecule) Allolactose, the inducer, is alternative form of lactose Fig. 7.4

9 Discovery of the lac Operon
1940s -1950s, Jacob & Monod studied metabolism of lactose by E. coli: biochemistry, genetics: cis/trans tests, antibodies to b-galactosidase, synthetic non-cleavable inducers Three enzyme activities / three genes were induced together by galactosides Constitutive mutants need no induction; genes are active all the time Merodiploids, partial diploid bacteria, constructed by conjugation of F’lac into F- lac cells, were critical to analysis of mutants, to decipher cis/trans relationships to define proteins, DNA sites

10 Effects of Regulatory Mutations: Wild-type and Mutated Repressors
Fig Lac I- mutations are recessive; repressor can act in trans to affect genes on both pieces of DNA

11 Effects of Regulatory Mutations: Wild-type and Mutated Operators (with Defective Binding)
Fig Lac Oc (constitutive) mutation is cis-dominant; affects only gene adjacent to mutant operator

12 Repressor-Operator Interactions
lac repressor binds lac operator (Filter-binding assay) [g32P]-labeled phage lambda DNA with lac operator was mixed with protein & filtered (protein binds, plus any bound DNA) Inducer IPTG removes repressor Mutated constitutive lac operator has lower affinity for lac repressor

13 Regulatory Mutations: Mutated Repressors (is) Bind Irreversibly; i-d repressors are dominant to WT
Fig. 7.5 mutant repressors act in trans

14 Mechanism of lac Repression still unclear
Initial binding in vitro: repressor does not block access by RNAP to lac promoter RNAP and repressor bind together to lac promoter Polymerase-promoter complex is in equilibrium with free polymerase and promoter Two hypotheses remain: RNAP can bind lac promoter in presence of repressor Repressor inhibits transition from abortive transcription to processive transcription Repressor, by binding to operator, blocks access by RNAP to adjacent promoter

15 There are 3 lac Operators
Major lac operator (O1) adjacent to promoter Auxiliary lac operators - upstream and downstream All operators required for optimum repression (Fig. 11) O1 alone produces only modest repression Repressor tetramer binds 2 operators to loop DNA Fig. 10

16 lac repressor tetramer has 2 DNA binding faces
Fig. 12 Ponzy Lu crystal structure with repressor, 21-bp O1 DNA

17 Catabolite Repression of lac Operon; cAMP
When glucose is present, lac operon relatively inactive Selection of glucose metabolism attributed to role of some breakdown product, catabolite Catabolite repression uses breakdown product of glucose to repress other catabolic operons Lack of glucose increases cAMP and relieves repression Fig. 13 5’-3’cAMP

18 Catabolite Activator Protein - CAP
cAMP added to E. coli cultures overcomes catabolite repression of lac operon: activates lac genes even in presence of glucose Positive controller of lac operon has 2 parts: cAMP Protein factor known as: Catabolite activator protein or CAP Cyclic-AMP receptor protein or CRP Gene encoding this protein is crp

19 CAP-cAMP Stimulates b-galactosidase synthesis
CAP-cAMP complex positively controls amount (activity) of b-galactosidase CAP binds cAMP tightly Mutant CAP does not bind cAMP tightly Compare activity and production of b-galactosidase Low activity with mutant CAP-cAMP Fig. 14 cell extracts of cultures

20 Mechanism of CAP Action
CAP-cAMP complex binds lac promoter tightly Mutants whose lac gene not stimulated by cAMP had mutations just upstream of lac promoter Ex. L1 deletion mutant Binding of CAP and cAMP to activator site helps RNAP form open promoter complex (RPo) CAP-cAMP activated operons have weak promoters The -35 boxes are unlike consensus sequence If promoters were strong, could be activated even when glucose is present Fig. 7.16

21 CAP Plus cAMP Action RPo does not form even if RNAP has bound DNA, unless CAP-cAMP complex is also bound Rifampicin used to prevent reinitiation Fig. 15

22 CAP-cAMP Recruits RNAP
Two steps: Formation of closed promoter complex RPo Conversion of RPo into open promoter complex CAP-cAMP bends target DNA about 100° when binds DNA red; CAP blue; cAMP thin pink; a green CAP contacts a subunit of RNAP (Fig. 17a)

23 CAP-cAMP Complexes bend promoter DNA
Measure protein binding to [g32P]-DNA using native PAG: Bent DNA runs slower than linear; Use fragments with CAP binding site (red) in different positions relative to restriction enzyme sites See which fragment runs slowest Fig. 18; experimental agreed with predictions

24 Model of CAP-cAMP Activation of lac Transcription
CAP-cAMP dimer binds to target site on DNA The aCTD (carboxy terminal domain of a subunit) of RNAP interacts with specific site on CAP Protein-protein interactions important, as are protein-DNA Strengthens binding between promoter and RNAP

25 7.2 The ara Operon of E. coli (ara CBAD)
ara operon encodes enzymes (genes B,A,D) required to metabolize sugar arabinose Catabolite-repressible operon Two ara operators: araO1 regulates transcription of control gene araC araO2 far upstream of promoter PBAD it controls CAP-binding site 200 bp upstream of PBAD promoter; yet CAP stimulates transcription Negative regulation mediated by AraC control protein

26 ara Operon Repression Loop
The araO2 operator controls transcription from PBAD promoter 250 downstream Data suggests DNA between operator and promoter loop out; proteins bind same face of DNA Mutant constructs changing spacing – 1 turn or ½ turn helix Fig. 20

27 AraC, the ara Control Protein
AraC, is both positive and negative regulator AraC has 3 binding sites: Far upstream site (-250), araO2 araO1 located between -106 and -144 araI is 2 half-sites, each bind one araC monomer: araI1 between -56 and -78 araI to -51 Fig. 7.21

28 Control of the ara Operon
Fig. 21 - glucose

29 AraC loops DNA by binding two operators
Add AraC to labeled minicircles with wt or mutant AraC binding sites -> looping makes more supercoiled and moves faster; At to, add excess unlabeled DNA of strong araI site; Electrophorese to see if still looped; mutants bind weaker Fig. 22

30 Positive Control of the ara Operon
Positive control is also mediated by CAP and cAMP CAP-cAMP complex attaches to binding site Pc upstream of araBAD promoter DNA looping would allow CAP to contact a subunit of RNAP, stimulate its binding to the promoter Fig. 7.21 - glucose

31 ara Operon Summary ara operon controlled by AraC protein
Represses by looping out DNA between araO2 and araI1 that are 210 bp apart Arabinose derepresses operon by binding AraC; AraC loosens attachment to araO2 and binds araI2 Breaking loop allows transcription of operon from PBAD CAP-cAMP stimulate transcription(bind upstream araI) AraC controls own synthesis by binding araO1 to prevent leftward transcription of araC gene from Pc Fig. 7.26

32 7.3 The trp Operon – anabolic - tryptophan
E. coli trp operon contains 5 genes (E,D,C,B,A) for enzymes to make amino acid tryptophan (anabolic) Anabolic enzyme synthesis typically on, and turned off by high level of product of pathway (repressed) Operon subject to negative control of transcription by repressor when high tryptophan levels The trp operator lies wholly within trp promoter Tryptophan helps trp repressor bind operator trp operon also exhibits attenuation of transcription

33 Negative Control of trp Operon
Without tryptophan no trp repressor exists, just inactive aporepressor If aporepressor binds tryptophan, changes conformation to gain high affinity for trp operator Aporepressor plus tryptophan makes trp repressor Tryptophan is corepressor Fig. 27

34 Attenuation of transcription in trp Operon extra level of control than just repressor-operator
Fig. 28: Attenuation is premature termination in 5’ UTR of trp E

35 Mechanism of Attenuation – trptophan high
Premature termination of operon’s transcript when product trp is abundant; Involves mRNA secondary structures: Hairpin plus string of U in leader signals termination, release of short RNA Fig. 29

36 Defeating Attenuation – tryptophan low
Prokaryotes start making protein before mRNA completely finished – only one cell compartment. If amino acid supply is low, ribosomes stall at tandem tryptophan codons in trp leader peptide stalled protein synthesis -> ribosome position influences mRNA folding Prevents formation of hairpin (part of transcription termination signal which causes attenuation) Fig Trp leader has tandem trp codons

37 Overriding Attenuation – low tryptophan
Ribosomes stall at tandem tryptophan codons in leader peptide Stalled ribosome position influences way mRNA folds Prevents formation of hairpin; hairpin part of transcription termination signal which caused attenuation if lots of trp and translation of leader. Fig. 7.32

38 7.4 Riboswitches Small molecules can act directly on 5’-UTRs of mRNAs to control gene expression Riboswitches –regions of 5’-UTRs that can alter structure to control gene expression after bind ligand Region that binds ligand is called aptamer Expression platform is other module in riboswitch: Terminator Ribosome-binding site Another RNA element that affects gene expression Operates by depressing gene expression Some work at transcriptional level Others can function at translational level

39 Model of Riboswitch Action: ribD gene for riboflavin synthesis
FMN binds to aptamer in called RFN element in 5’-UTR of ribD mRNA Binding FMN, base pairing in riboswitch changes to create terminator Transcription is attenuated Saves cell energy as FMN is product of the ribD operon Fig. 34 Model for B. subtilis ribD gene for riboflavin synthesis, the flavin mononucleotide FMN

40 Riboswitch Action: ribD gene
Excess FMN changes shape of 5’ mRNA leader Structure probed with RNase T1, OH- cleavage; -/+ FMN Fig. 7.33

41 Review problems 5. Describe and give results of experiment that shows lac operator is site of repressor binding. 21. Why does translation of the trp leader region not simply continue into the trp structural genes (trpE, etc.) in E. coli ? AQ 3. Consider E. coli cells each having one of the following mutations: Indicate effect each mutation on function of lac operon (assuming no glucose is present): a. mutant lac operator (Oc locus) that doesn’t bind repressor. b. mutant lac repressor (I-) that doesn’t bind operator c. Mutant lac repressor (Is) that doesn’t bind inducer d. Mutant lac promoter that doesn’t bind CAP-CAMP

42 Review problems 4. Why are negative and positive control of the lac operon important to the neergy efficiency of E. coli cells? 14. Diagram how arabinose relieves repression in the araBAD operon. Show whereAraC is located (a) in the absence of arabinose; (b) in the presence of arabinose and lack glucose. See Fig. 31. Because the his operon for synthesis of histidine operates much like trp operon. What might you predict about the sequence of amino acids encoded by the his leader?

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