Biology Lecture No. 14: The Lac Operon
Wednesday, February 29 , 2012
-Genes get transcribed by many polymerases and translated from multiple ribosomes.
-In order to determine the oldest ribosome, look for the direction of the RNA polymerase and note that
the free mRNA ends are the 5’ ends.
-Promoters are understood as signals composed of DNA.
The Lac Operon in E. coli:
-The lactose operon of E. coli was discovered by two scientists when they observed how E. coli had the
ability to get energy from lactose as result of the enzyme galactosidease.
-Galactosidease breaks apart lactose into glucose and galactose and can also form the potential product
allolactose (a lactose isomer) in a separate reaction with lactose.
-Normally, the activity of galactosidease as an enzyme is low, but not zero.
Inducible Wild Phenotype:
-In normal “wild type” phenotypes, adding lactose linearly expresses galactosidease (the Lac operon is
inducible by the presence of lactose).
-In mutant phenotype (Lac -), galactosidease follows constituent expression with or without the
presence of lactose (the Lac operon lacks its usual inducibility).
-In mutant phenotype (Lac Z ), galactosidease is never expressed (Lac operon completely lacks
A Lac Operon Simulation:
-The Lac Y structural gene codes for galactoside permease, a membrane-bound protein designed to
bring lactose into the cell.
-The Lac Z structural gene codes for galactosidease, which catalyzes lactose into glucose and galactose
-The Lac I structural gene codes for the lac repressor protein, which prevents transcription of the Lac
operon (Does not necessarily mean it will always be successful in stopping transcription).
- Both Lac Y and Lac Z share a common promoter, whereas Lac I is a separate gene with its own
promoter. The definition of an operon is several genes all under the same promoter (where in this case,
galactosidease is coupled with permease). -Expression of repressor proteins simply has an impact on the “turning on and off” of genes; in other
words it decreases the probability that they will be expressed.
-Lactose can be metabolized to fuel glycolysis as allolactose binds to and changes the conformation of
the repressor protein, rendering it unable to bind to the operator. The transcription of the Lac operon
concludes as much protein is synthesized in high lactose environments.
Mutant Case Study:
-If a mutant had a mutation in the Lac Z structural gene what would be the consequences? Well, there
would be no functional enzyme that degrades lactose and the concentration of lactose molecules rises.
The Lac Process And Negative Control:
-The Lac A structural gene codes for galactoside transacetylase, which transfers an acetyl group from
acetyl-Co-A to lactose.
-In the operator region of the Lac operon (the DNA binding site) there are loops that exist under
ordinary circumstances. When two molecules of repressor protein fuse and bind to the operator, they
stabilize this loop, which literally knocks off RNA polymerase in mid-transcription.
-This is known as negative control, the transcriptional regulation of the operon by repressor proteins.
CAP/cAMP And Positive Control:
-It is important to note that if glucose is made available, even in the presence of lactose, the cell will
prefer glucose as it would not have to express the lac operon nor its proteins.
-The CAP/cAMP binding site is a region where the Catabolyte Activator Protein (CAP) binds to stop
transcription as it notifies the cell that glucose is present in the environment (DNA often sense the
environment through the binding of proteins).