BLG 143 Study Guide - Final Guide: Lactose Permease, Lac Operon, Repressor

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17.3 Mechanisms of Negative Control: Discovery of the Repressor
Negative control occurs when a regulatory protein binds to DNA and shuts down
transcription.
Positive control occurs when a regulatory protein binds to DNA and triggers transcription.
lacZ and lacY genes in E. coli are controlled by both negative and positive control.
lacI gene produces an inhibitor that exerts negative control over lacZ and lacY.
This transcription inhibitor was called a repressor.
It was thought to bind directly to DNA near or on the promoter for lacZ and lacY.
The presence of lactose triggers transcription in normal cells by lactose interacting with
the repressor in a way that makes the repressor release from its binding site.
Lactose induces transcription by removing negative control.
Made E. coli cells that had functioning copies of the genes for β-galactosidase and lactose
permease but lacked a functional gene for the repressor.
These cells made β-galactosidase all the time.
But when they received a functioning copy of the repressor gene, β-galactosidase
production declined and then stopped.
The repressor codes for a protein that shuts down transcription.
However, if an inducer such as lactose was then added to the experimental cells, β-
galactosidase activity resumed.
Lactose removes the repressor.
The lacI gene codes for a repressor protein that exerts negative control on lacZ and lacY.
Lactose acts as an inducer by removing the repressing and ending negative control.
The lac Operon
Genes for β-galactosidase and lactose permease are controlled together.
Operon set of coordinately regulated bacterial genes that are transcribed together into
one mRNA.
The group of genes involved in lactose metabolism are lack operons.
lacA
Very close to lacY and lacZ and part of the same operon
Codes for enzyme transacetylase, function is protective in nature
Catalyzes reactions that allow certain types of sugars to be exported from the cell
when they’re too abundant
Lac operon regulation:
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Document Summary

17. 3 mechanisms of negative control: discovery of the repressor. Negative control occurs when a regulatory protein binds to dna and shuts down transcription. It was thought to bind directly to dna near or on the promoter for lacz and lacy. The presence of lactose triggers transcription in normal cells by lactose interacting with the repressor in a way that makes the repressor release from its binding site. Made e. coli cells that had functioning copies of the genes for -galactosidase and lactose. But when they received a functioning copy of the repressor gene, -galactosidase. However, if an inducer such as lactose was then added to the experimental cells, - production declined and then stopped. The repressor codes for a protein that shuts down transcription. permease but lacked a functional gene for the repressor. galactosidase activity resumed. The laci gene codes for a repressor protein that exerts negative control on lacz and lacy.

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