Negative regulatory proteins bind to operator sequences in the DNA and prevent or weaken RNA...
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Transcript of Negative regulatory proteins bind to operator sequences in the DNA and prevent or weaken RNA...
![Page 1: Negative regulatory proteins bind to operator sequences in the DNA and prevent or weaken RNA polymerase binding.](https://reader036.fdocuments.us/reader036/viewer/2022081514/56649d195503460f949ee8d3/html5/thumbnails/1.jpg)
Negative regulatory proteins bind to operator sequencesin the DNA and prevent or weaken RNA polymerase binding
![Page 2: Negative regulatory proteins bind to operator sequences in the DNA and prevent or weaken RNA polymerase binding.](https://reader036.fdocuments.us/reader036/viewer/2022081514/56649d195503460f949ee8d3/html5/thumbnails/2.jpg)
Most prokaryotic mRNA molecules are polycistronic, theyencode multiple genes. These genes are usually involved in the same biochemical event.
A single promoter controls the expression of these genes.
This functional unit of DNA is called an operon.
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A classical example oftranscriptional regulation
is lactose metabolismin E. coli.
Proteins required for lactose metabolism inE. coli are encoded by
the lac operon.
![Page 4: Negative regulatory proteins bind to operator sequences in the DNA and prevent or weaken RNA polymerase binding.](https://reader036.fdocuments.us/reader036/viewer/2022081514/56649d195503460f949ee8d3/html5/thumbnails/4.jpg)
The E. coli lac operon
lacI – encodes the Lac repressor proteinlacZ – encodes b-galactosidaselacY – encodes galactose permeaselacA – encodes transacetylaseO2 and O3 are pseudooperators
![Page 5: Negative regulatory proteins bind to operator sequences in the DNA and prevent or weaken RNA polymerase binding.](https://reader036.fdocuments.us/reader036/viewer/2022081514/56649d195503460f949ee8d3/html5/thumbnails/5.jpg)
The Lac repressor protein is thought to bind to the main operatorand one of the pseudooperators, forming a loop in the DNA.
![Page 6: Negative regulatory proteins bind to operator sequences in the DNA and prevent or weaken RNA polymerase binding.](https://reader036.fdocuments.us/reader036/viewer/2022081514/56649d195503460f949ee8d3/html5/thumbnails/6.jpg)
When lactose is present in high concentrations, the lactose metabolismgene products are needed in a cell. In the absence of lactose, the Lac repressor protein binds to the operator in the DNA, repressingtranscription. The Lac repressor, however, binds to allolactose, ametabolite of lactose, inducing a conformational change that abolishesbinding to the DNA operator sequence. Transcription is no longer repressed.
- allolactose transparent+ allolactose bold
![Page 7: Negative regulatory proteins bind to operator sequences in the DNA and prevent or weaken RNA polymerase binding.](https://reader036.fdocuments.us/reader036/viewer/2022081514/56649d195503460f949ee8d3/html5/thumbnails/7.jpg)
DNA binding proteins contain amino acids that hydrogen bond tofunctional groups in the major groove of DNA.
![Page 8: Negative regulatory proteins bind to operator sequences in the DNA and prevent or weaken RNA polymerase binding.](https://reader036.fdocuments.us/reader036/viewer/2022081514/56649d195503460f949ee8d3/html5/thumbnails/8.jpg)
DNA sequences recognized by regulatory proteins are often inverted repeats of a short DNA sequence. These repeats form a palindrome with two-fold symmetry about a central axis. Regulatory proteins are often dimeric. Each subunit binds to one strand of the DNA.
5’-TACGGTACTGTGCTCGAGCACTGCTGTACT-3’3’-ATGCCATGACACGAGCTCGTGACGACATGA-5’
central axis
![Page 9: Negative regulatory proteins bind to operator sequences in the DNA and prevent or weaken RNA polymerase binding.](https://reader036.fdocuments.us/reader036/viewer/2022081514/56649d195503460f949ee8d3/html5/thumbnails/9.jpg)
The Lac repressor protein
The Lac repressor is a tetramer of four identicalprotein subunits.
There are DNA-bindingdomains on each subunitshown in blue.
The allolactose binding domain (green) is connectedto the DNA binding domain through linker helices (yellow).
Tetramerization domains (red)form contacts between subunits.
![Page 10: Negative regulatory proteins bind to operator sequences in the DNA and prevent or weaken RNA polymerase binding.](https://reader036.fdocuments.us/reader036/viewer/2022081514/56649d195503460f949ee8d3/html5/thumbnails/10.jpg)
The Lac repressor protein
The Lac repressor is a tetramer of four identicalprotein subunits.
There are DNA-bindingdomains on each subunitshown in blue.
The allolactose binding domain (green) is connectedto the DNA binding domain through linker helices (yellow).
Tetramerization domains (red)form contacts between subunits.
![Page 11: Negative regulatory proteins bind to operator sequences in the DNA and prevent or weaken RNA polymerase binding.](https://reader036.fdocuments.us/reader036/viewer/2022081514/56649d195503460f949ee8d3/html5/thumbnails/11.jpg)
The DNA binding domains of the Lac repressor contain a helix-turn-helix motif, a structure critical for the interaction of many
proteins with DNA.
helix
turnhelix
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Lac repressor protein
(lacI)
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Lac repressor bound to DNA
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Lac repressor bound to DNA