BIO282 Lecture Notes - Lecture 17: Growth Medium, Trp Operon, Permease
cAMP-CAP complex acts as a positive regulator
The catabolite activator protein (CAP) binds to the promoter of the lac operon to stimulate
transcription. (see Fig 16-13)
• CAP must complex with cAMP (adenosine-3`, 5`-cyclic monophosphate) before binding to the
promoter of the lac operon.
• The binding of cAMP–CAP to the promoter activates transcription by facilitating the binding of
RNA polymerase.
• Levels of cAMP are inversely related to glucose: low glucose stimulates high cAMP; high
glucose stimulates low cAMP.
• In presence of glucose the lac operon remains switched off even if lactose is available to cells.
The catabolite activator protein (CAP) binds to the lac promoter and stimulates transcription.
CAP - catabolite activator protein - activates catabolism of other compounds
Gene regulation by DNA bending
The catabolite activator protein (CAP) binds to the lac promoter and stimulates transcription.
• Active CAP recognizes a specific sequence of DNA for binding
• CAP binding induces DNA bending.
• The mechanism of bending is to introduce a sharp kink within the binding sequence.
• Kinks in each copy result in a 90O bend.
• CAP functions by interacting with RNAP (α subunit).
Gene regulation by DNA looping
The ara operon has
three structural genes araB, araA and araD; a regulatory gene araC;
four regulatory regions araO1, araO2, araI1 and araI2.
How do araO2 and CAP binding sites affect transcription from PBAD?
In the absence of arabinose
• The regulatory protein AraC binds to the regulatory regions I1 and O2
• AraC bound to I1 and O2 interact with each other causing DNA looping
• In this form no transcription can take place from PBAD and PC
• Which type of regulator is AraC?
o Here AraC is a negative regulator
In the presence of arabinose
• Arabinose interacts with the regulatory protein AraC
• AraC bound to arabinose has different affinity to its binding sites.
• Binding of AraC at I2 and CAP at the CAP binding site are required for transcription through the
PBad promoter.
• Which type of regulator is AraC?
o Here AraC is a positive regulator
Document Summary
Levels of camp are inversely related to glucose: low glucose stimulates high camp; high glucose stimulates low camp. In presence of glucose the lac operon remains switched off even if lactose is available to cells. The catabolite activator protein (cap) binds to the lac promoter and stimulates transcription. Cap - catabolite activator protein - activates catabolism of other compounds. The ara operon has three structural genes arab, araa and arad; a regulatory gene arac; four regulatory regions arao1, arao2, arai1 and arai2. In the absence of arabinose: the regulatory protein arac binds to the regulatory regions i1 and o2, arac bound to i1 and o2 interact with each other causing dna looping. In this form no transcription can take place from pbad and pc: which type of regulator is arac, here arac is a negative regulator. Pbad promoter: which type of regulator is arac, here arac is a positive regulator.
