Another small presentation.

1. Sample Pathway for Control of
Promotor (RNA Tryptophan (trp) Expression
polymerase binding
site)
Enzyme 1
Gene 1
Enzyme 2
Gene 2
Enzyme 3
DNA
Gene 4
not a s na t
i l
r
not pr cs na t
i i
r
Gene 3
Gene 5
mRNA
(transcription unit)
Enzyme 4
Each enzyme
catalyzes the
next series of
reactions
necessary for
tryptophan
production
Enzyme 5
trp
2 Negative Feedback
shut-off systems

2. Basic Vocabulary
mRNA Transcription Unit:
- grouping of mRNA genes with related
function
- 2 advantages ~
1. organization
2. easy regulation – one “on/off” switch
can be used to control an entire cluster
of related genes

3. Basic Vocabulary
Operon:
- an entire gene-regulating system in
PROKARYOTES, which includes the
operator, promotor, and all corresponding
genes

4. Basic Vocabulary
Let’s take a closer
look at the
tryptophan (trp)
operon…an
example of a
“repressor” system
Operator:
- “on-off” switch to genes
- controls the access of RNA polymerase
to genes
- found within the promotor region or
between the promoter and gene-encoding
regions

5. Overview controls the Trp Operon
Trp operonof Repressor production
Components
of the amino acid tryptophan
r. Trp
promotor
Repressor
5 Genes needed for
Gene
tryptophan production
rp. RNA Pol. o. Operator
(always
binding site
“on”)
r.
rp.
o.
Gene 1 Gene 1Gene 3 Gene 4 Gene 5
RNA polymerase
mRNA transcription unit
mRNA for trp
repressor
Enz 1
Trp repressor protein
translated in its
“inactive” form
Enz 2
Enz 3
Enz 4
Enz 5
trp

6. How is the trp operon turned off
once enough trp is made?
Trp repressor
gene
RNA pol
binding site
operator
trp
trp
trp
trp
Inactive trp
repressor
protein
trp
Increased concentration of trp (corepressor) increases chances of trp
binding to allosteric site of inactive trp
repressor

7. How is the trp operon turned off
once enough trp is made?
RNA polymerse,
therefore is
physically blocked
from transcribing
genes for trp
Active trp repressor
can now bind to
operator

8. Why is this a repressor system?
The trp operon is a repressor operon, meaning
gene expression of the operon is repressed
by the presence of the co-repressor,
tryptophan.

9. Watch this animation to fully
understand and review this operon,
then take the on-line quiz together
as a class
http://bcs.whfreeman.com/thelifewire/content/chp

10. Overview of Inducible Lac Operon
The Lac operon controls the production of
the ß-galactosidase, an enzyme that
catalyzes the hydrolysis (break-down) of
lactose into glucose and galactose.
This is an inducible operon, meaning gene
expression ß-galactosidase is stimulated
by the presence of an co-inducer, lactose.

11. Inducible Lac operon
promotor
I. Lac
Inducer
I.
mRNA for
inducer
protein
inducer
protein in
“active”
form
crp: helps
RNA pol.
to bind
rp: RNA
pol.binding
site
crp.
rp.
o. operator
Genes that
promote ßgalactosidase
production
o.
RNA pol.
blocked from ßgalactosidase
transcription
without lactose
co-inducer

12. TWO regulatory mechanisms used
to turn on lac operon
1) Presence of lactose as a co-inducer
** reason?
ß-galactosidase is not needed unless lactose
needs to be broken down
2) Low amounts of glucose
** reason?
Recall that lactose breaks down into glucose and
galactose. Low glucose levels signals the cell
for more lactose to be broken down.
Both conditions must be met for the lac operon to turn on.

13. TWO regulatory mechanisms used
to turn on lac operon
RNA polymerase can bind
only with the help of CRP
transcription factor
RNA pol.
AlloLactose
cAMP
CRP Transcription
Factor
2. cAMP attaches to CRP
transcription factor when
glucose is scarce
1) Allolactose coinducer attaches to
allosteric site of induce
protein, inactivating it

14. Lac Operon Animation
Watch this animation to fully understand and
review the lac operon!
http://highered.mcgraw-hill.com/olc/dl/120080/bi

15. How are genes controlled in
eukaryotes?
1) Regulation of Chromatin Structure
2) Pre and post Transcriptional Regulation
3) Pre and Post Translational Regulation

16. Regulation of Chromatin Structure
N-terminus (amino group) of histone proteins face outwards
from nucleosome
Tails are thus able to be modified chemically

17. Regulation of Chromatin Structure
Histone Acetylation – neutralizes (+) charges on
•• Methylation  Promotes condensation
tails, which prevents binding to condensation, if
• Phosphorylation  can prevent adjacent
nucleosome  loose chromatin structure
phosphorylation is adjacent to methyl group
results, allowing for increased transcription

18. Pre-Transcriptional Regulation
• Similar to methods used in bacterial
operons, using proteins that inhibit or
promote binding of RNA pol.
– Distal and Proximal Control Elements
– Proteins involved include:
• Transcription factors
• Activators
• Mediator Proteins

20. Post-Transcriptional Regulation
RNA Processing
– differential/alternative splicing can produce
different 20 mRNA transcript
Differential splicing redefines
which RNA segments are
considered introns and which
are exons

21. Post-Transcriptional Regulation
• Time of mRNA degradation can vary

22. Pre-Translational Regulation
Initiation of Translation
- can be blocked by regulatory proteins
that prevent ribosome binding
- shortened polyA tails in mRNA prevents
translation (polyA tails can be added
during appropriate time)
- global regulatory control of all mRNAs in
cell

23. Post-Translational Regulation
• During protein processing, folding
• Timing of protein degredation can vary
• Proteasomes degrade proteins that are
tagged by ubiquitine molecules