Daily Newsletter January 30, 2012
Today's Topic: CRP and AraC
CRP stands for cAMP receptor protein, and it is involved in the regulation of the Lac operon. Since you only want the lac operon to operate when there is low glucose, you use a CRP to prevent transcription when you are living in an environment of high glucose concentrations. As with any regulatory system, you have to have a signal for when to remove CRP.
First thing to remember: Your book focuses on CRP in terms of the Lac operon, but CRP is a general regulator. It blocks multiple genes and operons from expression when you live in adequate levels of glucose. When you go toward starvation, in this case low glucose, you need to remove all of the CRPs on DNA. So, glucose is are primary signal. The cell though hates to use glucose as a signal, so we will transduce this primary signal into a secondary messager: cAMP. You should have seen cyclic AMP before, and should be familiar with it being used as a secondary messager. So, how does CRP work?
Remember that you have a double promoter (-10, -35). There are some promoters that cause the RNA polymerase to bind and then get stuck, preventing transcription (the polymerase instead pops off). Such is the case with the promoter for the Lac operon. CRP when it binds with cAMP (CRP-cAMP) can bind to a segment of DNA at -60 from the Lac Operon. The CRP-cAMP complex forces the DNA to bend, which causes the CRP-cAMP complex to come into contact with the RNA polymerase. This induces a molecular stress that forces the RNA polymerase to start working. As an analogy, the CRP-cAMP complex gives the stalled/stuck RNA polymerase a push to get moving.
The AraC regulator controls the Arabinose operon. This is a feedback regulation, but has some interesting complexities that show the diversity of bacterial genetics. First, the coding region of the operon is transcribed in one direction, but the coding region of the regulator is transcribed in the opposite direction (Question: How do you do that?). AraC (the regulator) creates a dimer protein (two protein units) that have DNA binding domains. When arabinose is absent, the AraC protein dimer binds to DNA, causing it to curve (or loop). In this form, there is no interaction between AraC and RNA Polymerase; no transcription occurs. When Arabinose is present, the dimer binds to arabinose and experiences as configurational change (it changes shape). The AraC dimer can then interact with RNA Polymerase. As with CRP-cAMP, it gives RNA Polymerase a push to get it to start transcribing. This is a basic description of this regulatory system, and leaves out the action of CRP-cAMP as well as not giving specific gene sequences effected. Still, it should give you a basic idea about this mechanism.
Daily Challenge: Gene Regulation
In today's blog, choose either the CPR-cAMP or AraC regulatory mechanism and describe it in detail (if you feel ambitious, do both). This must be in your own words, and go more in depth than the discussion above. This is a great blog in which to add images (just remember to discuss the images).
CRP stands for cAMP receptor protein, and it is involved in the regulation of the Lac operon. Since you only want the lac operon to operate when there is low glucose, you use a CRP to prevent transcription when you are living in an environment of high glucose concentrations. As with any regulatory system, you have to have a signal for when to remove CRP.
First thing to remember: Your book focuses on CRP in terms of the Lac operon, but CRP is a general regulator. It blocks multiple genes and operons from expression when you live in adequate levels of glucose. When you go toward starvation, in this case low glucose, you need to remove all of the CRPs on DNA. So, glucose is are primary signal. The cell though hates to use glucose as a signal, so we will transduce this primary signal into a secondary messager: cAMP. You should have seen cyclic AMP before, and should be familiar with it being used as a secondary messager. So, how does CRP work?
Remember that you have a double promoter (-10, -35). There are some promoters that cause the RNA polymerase to bind and then get stuck, preventing transcription (the polymerase instead pops off). Such is the case with the promoter for the Lac operon. CRP when it binds with cAMP (CRP-cAMP) can bind to a segment of DNA at -60 from the Lac Operon. The CRP-cAMP complex forces the DNA to bend, which causes the CRP-cAMP complex to come into contact with the RNA polymerase. This induces a molecular stress that forces the RNA polymerase to start working. As an analogy, the CRP-cAMP complex gives the stalled/stuck RNA polymerase a push to get moving.
The AraC regulator controls the Arabinose operon. This is a feedback regulation, but has some interesting complexities that show the diversity of bacterial genetics. First, the coding region of the operon is transcribed in one direction, but the coding region of the regulator is transcribed in the opposite direction (Question: How do you do that?). AraC (the regulator) creates a dimer protein (two protein units) that have DNA binding domains. When arabinose is absent, the AraC protein dimer binds to DNA, causing it to curve (or loop). In this form, there is no interaction between AraC and RNA Polymerase; no transcription occurs. When Arabinose is present, the dimer binds to arabinose and experiences as configurational change (it changes shape). The AraC dimer can then interact with RNA Polymerase. As with CRP-cAMP, it gives RNA Polymerase a push to get it to start transcribing. This is a basic description of this regulatory system, and leaves out the action of CRP-cAMP as well as not giving specific gene sequences effected. Still, it should give you a basic idea about this mechanism.
Daily Challenge: Gene Regulation
In today's blog, choose either the CPR-cAMP or AraC regulatory mechanism and describe it in detail (if you feel ambitious, do both). This must be in your own words, and go more in depth than the discussion above. This is a great blog in which to add images (just remember to discuss the images).
