BIOL 142 Chapter 18.3 : Negative Control of Transcription
18.3%Negative%Control%of%Transcription%
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Two!ways!transcription!can!be!regulated!
• Negative(control!occurs!when!a!regulatory!repressor!protein!binds!to!DNA!and!shuts!down!
transcription!
o Negative(control:!a!type!of!regulation!that!works!by!slowing!an!ongoing!process;!in!gene!
transcription,!the!action!of!a!regulatory!protein!to!shut!down!transcription!by!binding!to!
DNA!in!or!near!the!gene!
o Repressor:!in!bacteria,!a!protein!that!binds!to!an!operator!sequence!in!DNA!to!prevent!
transcription!when!an!inducer!is!not!present!and!that!comes!off!DNA!to!allow!transcription!
when!an!inducer!binds!to!this!protein!
• In!eukaryotes,!a!protein!that!binds!to!a!silencer!sequence!in!DNA!to!prevent!or!reduce!
gene!transcription!
o Negative(control:!regulatory!protein!shuts%down!transcription!
!
• Positive(control!occurs!when!a!regulatory!activator(protein!binds!to!DNA!and!triggers!
transcription!
• Positive(control:!a!type!of!regulation!that!works!by!speeding!up!a!process;!in!gene!
transcription,!the!action!of!a!regulatory!protein!to!trigger!expression!by!binding!to!DNA!in!or!
near!the!gene!
• Activator:!a!protein!that!binds!to!a!DNA!regulatory!sequence!to!increase!the!frequency!of!
transcription!by!RNA!polymerase!
• Positive(control:!regulatory!protein!triggers!transcription!
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Car!model!
• lacZ!and!lacY!genes!are!under!negative!control!
• Like!pressing!the!brake!pedal!
• lacI!produces!a!repressor!protein!that!exerts!negative!control!over!lacZ!and!lacY!by!binding!
directly!to!DNA!at!or!near!the!promoter!for!the!2!genes!
• Binding!the!repressor!blocks!the!action!of!RNA!polymerase!by!preventing!initiation!or!
binding!of!DNA!polymerase!to!the!promoter!
• lacI!is!under!positive!control!
• Like!pressing!the!gas!pedal!
• Represses!the!transcription!of!lacZ!and!lacY!
• Constitutive!transcription!is!observed!in!these!mutants!because!a!functional!repressor!is!
absent!–!no!brake!
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Genes!involved!in!lactose!metabolism!are!under!negative!control!
• lacI(gene(codes(for(a(repressor(protein(that(exerts(negative(control(on(lacZ(and(lacY!
• Lactose(acts(as(an(inducer(by(causing(the(repressor(to(release(from(DNA(and(ending(negative(
control!
• The!plasma!membrane!and!galactoside!permease!are!shown!as!a!reminder!that!lactose!comes!
from!outside!the!cell!
• Repression!of!lac!genes!is!never!complete,!so!there!is!always!some!galactoside!permease!to!
transport!lactose!into!the!cell!and!begin!induction!of!gene!expression!
• Lactose!absent!inside!cell,!repressor!active:!
• Receptor!binds!to!DNA!
• Transcription!is!blocked!
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• Lactose!present!in!cell,!receptor!inactive:!
• Lactose!(the!inducer)!binds!to!receptor!
• Repressor!releases!from!DNA!!
• Transcription!occurs!
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• Lactose!present!or!absent!inside!cell,!mutant!repressor!gene!
• Transcription!occurs!