The lac Operon of E. Coli
AMATH 882
Lecture 11, Feb. 12, 2013
The lac operon offers one of the simplest genetic
circuits: an autoregulatory circuit (in which the
product regulates the rate of expression of its
own gene).
An operon is a set of genes found in sequence
which share a regulatory region, and so are
coordinately regulated.
trp operon:
http://www.aw-bc.com/mathews/ch26/fi26p33.gif
The lac operon codes for three proteins:
-galactosidase (produced by gene lacZ)
- galactoside permease (produced by gene lacY)
- galactoside transacetylase (produced by gene
lacA)
http://www.science.siu.edu/microbiology/micr460/460%2520Pages/Lac.operon.html&h=313&w=478&sz=5&tbnid=NrkEQdMXcWgJ
:&tbnh=82&tbnw=125&start=14&prev=/images%3Fq%3Doperon%2Blac%26hl%3Den%26lr%3D
Function of the lac operon:
E. coli’s preferred energy source is the sugar glucose.
When glucose is available, the cell maintains only very
low levels of enzymes needed to metabolize other
substrates (e.g. other sugars). In particular, in such a
case there are relatively few copies of the enzyme galactosidase which breaks down lactose (milk sugar) to
galacatose and glucose.
-galactosidase
http://www.indiana.edu/~ensiweb/lessons/tp.2.gif
Production of -galactosidase is kept low by a
repressor bound to the lac operon.
http://bio.winona.msus.edu/berg/3
08s04/Lec-note/21-new.htm
However, when
switched to a lactoserich, glucose-poor
environment, the cell
takes up lactose
which is isomerized
by -galactosidase to
allolactose (this is a
second reaction
catalysed by galactosidase)
-galactosidase
http://www.biochem.arizona.edu/classes/bioc471/pages/Lecture4/Lecture4.html
Allolactose binds to the repressor and reduces its
affinity for the regulatory site (terminology:
allolactose sequesters the repressor away from
the regulatory site).
http://bio.winona.msus.edu/berg/3
08s04/Lec-note/21-new.htm
Consequently, the cell begins transcription of
the lac operon. (This response happens on a
timescale of minutes.) The result: increased
levels of -galactosidase, permease, and
transacetylase.
http://bio.winona.msus.edu/berg/3
08s04/Lec-note/21-new.htm
The permease mediates increased
transport of lactose into the cell,
while increased levels of galactosidase lead to increased
production of allolactose (setting up
a positive feedback) and also
production of galactose and glucose
(an energy supply). (The
transacetylase is not essential for
lactose metabolism. It's function as
part of the operon is not understood) http://bio.winona.msus.edu/berg/308s04/Lec-note/21-new.htm
-galactosidase
http://www.indiana.edu/~ensiweb/lessons/tp.2.gif
Remark: this is not the whole story.
Transcription of the genes in the
operon also requires the presence
of an inducer: catabolic gene
activiting protein (CAP) (also
known as cAMP receptor protein
(CRP)) which is constituitively
present in the cell, but becomes
active when bound to cyclic
adenosine monophosphate
(cAMP). cAMP is a "hunger signal"
which varies inversely with glucose
levels in the cell.
http://bio.winona.msus.edu/berg/3
08s04/Lec-note/21-new.htm