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Home >>Regulation of Gene Expression - in Prokaryotes >> Positive Control of Lac Operon
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Positive Control of Lac Operon - Interestingly, if bacteria encounter two sugars, e.g. glucose and lactose, then E. coli cells will keep the lac operon turned down as long as glucose is present. It looks appropriate if glucose is still available, because, E. coli cells metabolize glucose more easily than lactose, it would therefore be wasteful for them to activate the lac operon in the presence of glucose.

The selection of glucose metabolism against the use of other energy sources has long been attributed to the influence of some breakdown product or catabolite of glucose. It is therefore known as catabolite repression.

The ideal positive controller of the lac operon would be a substance that senses the lack of glucose and responds by activating the lac promoter so that RNA polymerase could bind and transcribe the structural genes. One substance that responds to glucose concentration is a nucleotide called cyclic AMP (cAMP). As the level of glucose drops, the concentration of cAMP rises.

Then CAP (catabolite activator protein) binds to cAMP and the resulting complex binds to the lac control region and helps RNA polymerase to bind there. X-ray diffraction studies of the CAP/cAMP/promoter complex has shown that when CAP/cAMP binds, it causes the DNA to bend by 90°.

This bending apparently helps RNA polymerase to separate the two DNA strands, forming an open promoter complex. CAP/cAMP may also stimulate transcription by direct protein­ protein contact between CAP and RNA polymerase.

 

  

 
 

 
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