(Transcribed from Dr. Cadilla’s lecture, 24 Mar 2000 by Brian Buschman)
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The control of what genes are expressed involves many different units and control sequences. The basic, complete regulatory unit of a set of genes in eukaryotes is the operon which contains the following units in a nice group.
Gene expression includes induction and repression which a gene’s expression is increased and decreased respectively.
The lactose (lac) operon is used as a model of eukaryotic gene expression. The lac operon contains three adjacent structural genes:
1) LacZ codes for b-galactosidase which cleaves lactose.
2) LacY codes for the sugar transporter that brings lactose into the cell.
3) LacA codes for b-galactosidase transacetylase.
In the absence of glucose the lac operon is activated and the above three units are produced. It is called polycistronic because the one operon has multiple structural genes.
When lactose in present the inducer binds to the operator which causes removal of the repressor. The gene is naturally “turned off” but when this happens it is “turned on.”
The tryptophan operon has five structural genes which code for enzymes that synthesize tryptophan. The tryptophan operon acts differently from the lac operon in that it is “always on” unless it is repressed. When tryptophan is sufficient tryptophan will act as a corepressor and bind with the repressor to the operator located in the middle of the promoter. This blocks the binding of RNA polymerase and prevents gene expression.
One unique feature of the tryptophan operon is that the last three structural genes produce proteins that do not code for any tryptophan residues so that tryptophan can be synthesized even in cases of severe tryptophan shortage.
Unlike prokaryotes, eukaryotes rarely have clusters of genes like the operon. The regulatory sequence may be a thousand bases away. The promoters are still right next to the structural gene but the regulator may have to function by binding the regulatory unit and then looping the DNA so that it is brought close to the promoter.
Enhancers are located upstream. When the inducer binds to the enhancer the DNA will form a loop and bring the enhancer to the promoter. This wil cause the RNA pol II to start doing it’s job.
Response elements are promoters that automatically respond by expressing their gene in response to a stimulus. Examples include heat shock elements and glucocorticoid response elements. There are a number of response elements for helping take care of dangerous effects on the cell.
The lac operon is only active in the absence of glucose and in the presence of lactose. When lactose is high and the catabolic activator protein (CAP) and cAMP binds the CAP it exerts positive control on the lac operon causing expression. Mutations effect this process. There are no need to memorize then but to understand the process and logically deduce what must be broken by the given mutation.
Helix-turn-helix units are made of two a-helices separated by a b-turn. It fits into the major groove of DNA and blocks the work of a polymerase.
Zn-fingers consist of repetitive sequences of Cys with some His that coordinate Zn ions. These units are good for binding steroid and thyroid hormones and are receptors for these molecules.
Leucine zippers are made of two a-helices. One side is basic which binds DNA and one side is made of lots of leucine and binds to the leucine rich region of another leucine zipper molecule. They function to bind two units together.
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