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Name: Robert
Status: Student
Age: N/A
Location: N/A
Country: N/A
Date: September 2004


Question:
"RNA polymerase must be able to recognize the beginning of a gene so that it knows where to start synthesizing an mRNA. It is directed to the start site of transcription by one of its subunits' affinity to a particular DNA sequence that appears at the beginning of genes. This sequence is called a promoter."

My question is now more about the flow of what is going on. You have this RNA polymerase swimming around. It finds its way to the DNA. (How does it find it's way to the DNA or is there an RNA polymerase attached to the DNA at each promoter location?) The DNA is made up of many genes. The RNA polymerase uses the promoter to find its way to the gene that will be transcripted. But, with so many genes on the DNA, what determines which which gene's promoter the RNA polymerase is going to use? Are there separate promoter's for each gene? If so, are they stored sequentially?



Replies:
The way RNA polymerase molecules "find" the promoter is presumably by random diffusion (thermal motion). I don't know about eucaryotic cells, but in bacteria, RNA polymerase can bind to any promoter that is not being blocked by the corresponding repressor protein molecule. For inducible operons like beta-galactosidase, the repressor is active when there is no lactose present; when lactose is present, it binds to the repressor and inactivates it so transcription can occur. In the case of biosynthetic operons like tryptophan, when tryptophan is present the repressor is activated thus shutting down transcription, and when tryptophan is scarce, the repressor is inactive and transcription can take place. As you suggested there are separate promoters for each gene.

Hope this helps,

Regards, Ron Baker, Ph.D.



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