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Operon Theory
lacI: lac repressor
Lac operon and structural genes
lacI: lac repressorlacZ: β-galactosidase, LacY: β-galactoside permease and LacA: thiogalactoside transacetylase (also known as galactoside O-acetyltransferase) Lac repressorCatabolite activator protein(CAP)
Regulation by cyclic AMP
1. cya: adenylate cyclase which produces cyclic AMP
2. crp: catabolite activator or cAMP receptor protein(CRP)
Isopropyl-β-D-thio-galactoside (IPTG): It is not a substrate for β-galactosidase, so that it cannot be metabolized by E. coli
Phenyl-β-D-galactose (phenyl-Gal): is a substrate for β-galactosidase, but does not inactivate repressor and so is not an inducer
Lactose Analogues
ortho-Nitrophenyl-β-galactoside(ONPG): a colorimetric and spectrophotometric substrate for detection of beta-galactosidase
Allolactose (β1->6): isomer of lactose and the inducer of the lacoperon
Analogues
Classification of regulatory mutants: complementary test
lac Oc mutant, cis-dominant
lac I mutant
dominant
β-galactosidase transacetylase
Genotype -IPTG +IPTG -IPTG +IPTG Conclusion
I+Z+A+ <0.1 100 <1 100 Inducible
I+Z-A+ <0.1 <0.1 <1 100 lacZ encodes
β-galactosidase
I-Z+A+ 100 100 90 90 Constitutive
Table Phenotypes of repressor mutants
I-Z+A+ 100 100 90 90 Constitutive
I+Z-A+ /F' I-Z+A+ <0.1 100 <1 200 I+ >I- in trans
IsZ+A+ <0.1 <1 <1 <1 Noninducible
IsZ+A+ /F' I+Z+A+ <0.1 1 <1 1 Is >.I+ in trans
Trp operon and attenuation
Three enzymes required to convert chorismic acid into tryptophan:Three enzymes required to convert chorismic acid into tryptophan:1) Anthranilate synthetase
component I - encoded by trpEcomponent II - encoded by trpD
2) N-(5'-phosphoribosyl)-anthranilate isomerase/Indole-3-glycerol phosphate synthase - encoded by trpC
3) Tryptophan synthaseb-subunit - encoded by trpBa-subunit - encoded by trpA
trpL : coding for a short oligopeptide (trpL) which functions in attenuation.
trpR : Trp repressor protein . Unlike the lac operon, trpR is not adjacent to the operon. The structural genes are located at minute 28 on the E. coli map; trpR is located at minute 100. The TrpR repressor actually regulates the expression of two other operons in addition to the trpoperon.
TrpR protein is unable to bind to the operator by itself. As long as there is insufficient tryptophan in the cell, it will remain so. However, once the is insufficient tryptophan in the cell, it will remain so. However, once the level of tryptophan builds up, then the Trp repressor will block further transcription of the operon and, as a result, the synthesis of the three enzymes will decline.
Trp operon Attenuation
FIG. 1. Construction of pHGH807tacI containing the trp-lachybrid promoter tacI.
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