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Promoter Determination
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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