Chapter 16 – Regulation of Gene Expression - Prokaryotes
I. Introduction
A. Regulation takes place in response to the external and internal environments.
B. Types of gene expression
1. Early 1900s - recognized that some genes are always expressed (constitutive).
2. Others only expressed under certain conditions
Only when a certain condition is present (inducible) or not expressed when certain conditions are
present (repressible).
c. May be positive or negative:
1. Negative - expression takes place unless specifically turned off by a regulator. The regulator
interferes with the process of transcription.
2. Positive - gene expressed only when stimulated (turned on) by a regulator. The regulator is
necessary for transcription.
II. Prokaryotes - regulation of gene expression - some examples:
A. The operon model 1. Several genes related in function are clustered together near a single regulatory region
2. regulation comprised of two elements:
cis-acting element - regulatory sequence upstream of the genes
trans-acting element - molecule that binds to the regulatory region to control transcription
B. The lac operon (Figure 16.1) - proposed by Jacob & Monod (1960)
1. An example of an inducible system under negative control
2. Features
a. structural genes (responsible for lactose digestion), regulatory region and repressor gene
- lactose acts as the inducer
3. Figure 16.3 - lac genes transcribed as polycistronic mRNA
4. Mechanism of Lac operon - Figure 16.5
a. Repressor gene - produce protein (repressor molecule) that binds with operator gene and also
has a binding site for lactose
Repressor = allosteric - binds with another molecule - changes conformation of repressor
b. Without lactose - RNA polymerase can't transcribe the genes because it is blocked by the
repressor molecule.
c. With lactose - lactose binds with the repressor molecule, repressor no longer binds to the
operator gene (falls off). Allows transcription.
d. Negative control - gene expression would normally take place unless it is specifically blocked
5. Lac operon mutants - impact on gene expression - Figures 16.6 & 16.7
C. Example of positive control - catabolite-activating protein (CAP)
1. Also controls lac operon on the basis of presence/absence of glucose:
Represses operon when glucose is present
2. Figure 16.8 - CAP binds with the promoter of lac operon (under low glucose concentrations)
to facilitate the binding of the RNA polymerase (allowing transcription).
When glucose is present, it indirectly inhibits the binding of CAP to the promoter through its
influence on cyclic AMP levels, thus shutting off the lac operon. (cAMP is necessary for CAP
binding to promoter)
Chapter 16 – Regulation of Gene Expression - Prokaryotes
3. Positive control - regulator molecule (CAP) is necessary for transcription to take place.
D. Example of a repressible gene system, also with negative control
Tryptophan operon in E. coli. - Figure 16.12
1. Tryptophan synthesis controlled by a 5 genes (if try present, don’t need to synthesize it)
2. Model - repressor molecule is present but inactive. In this case the genes are transcribed.
Needs a co-repressor (tryptophan) to bind with it to produce a fully functional repressor
molecule. When activated, the repressor molecule blocks transcription of these genes.
E. Animation review of lac operon
http://www.sumanasinc.com/webcontent/anisamples/majorsbiology/lacoperon.html