Gene Regulation –II
Fine Control of Prokaryotic Transcription
By Professor MinDong Song, Ph.D
Division of Life Science,
Konkuk University, Chungju, Korea
강의 내용
I. 서 론 : Review & Summary of lac Operon
II. Other Operons than lac operon
1) Galactose(gal) operon : catabolism
2) Arabinose(ara) operon: catabolism
3) Tryptophan(trp) operon : anabolism
Lac operon map
Regulatory
sites
i
pi
CAP
p
Structural genes
o
z
y
Galactoside transacetylase
Repressor
Lactose permease
β-galactosidase
CAP: catabolite activator protein
P : promoter(RNA polymerase binding site)
O: operator(Repressor binding site)
a
Negative Regulation of lac operon
i
Pi P
z
y
a
유도물질(inducer)이 없을 때
억제 단백질(repressor)이 operator에
결합하여 lac operon의 전사를 방해한다
Glucose
Lactose(inducer)
+
-
억제유전자 mRNA
억제 단백질
i
Pi P O
z
y
a
유도물질(inducer)이 있을 때
Glucose
Lactose(inducer)
억제유전자 mRNA
베타-갈락토오스 분해효소
락토오스 침투효소
억제 단백질
Allo-lactose
아세틸 전이 효소
(불활성화 억제 단백질)
+
Positive Regulation of lac operon
RNA
중합효소
Promoter 결합
CAP
글루코오스
농도가 낮을 때
P
CAP
O
lac z
lac y
lac a
mRNA
+
ribosome
베타-갈락토오스 분해효소
락토오스 침투효소
cAMP
아세틸 전이 효소
Summary of lac operon
1. Negative Regulation by Repressor
1-1. In the absence of inducer(lactose)
→ turn off lac operon by repressor
1-2. In the presence of inducer
→ turn on lac operon by inducer-repressor
2. Positive Regulation by Activator
No Glucose → increase cAMP conc. →cAMP binds to CRP(or CAP)
→ cAMP•CRP complex → enhance RNA polymerase to Promoter →
Increase Transcription
Animation for lac operon
• Animation for lac operon
Galactose operon
• The galactose operon of E. coli is also an inducible system, regulated by
cAMP-CRP and a repressor.
• The gal operon has three cistrons-galE, galT, and galK, which encode
enzymes that metabolize galactose-and is transcribed into polycistronic mRNA
molecules.
• The operon contains two promoters, P1 and P2.
• The cAMP levels in the cell influence the gal operon in an interesting way:
The cAMP-CRP complex binds to a noncoding DNA site in the operon and
regulates transcription from two promoters in opposite fashions
• cAMP-CRP activates transcription from promoter P1, but inhibit transcription
from promoter P2
• This mechanism of dual control insures that the galactose-metabolizing
enzymes are synthesized whether the cells are growing in the presence of
glucose or in another carbon source.
Regulation of Transcription of the gal operon
In the absence of cAMP-CRP, gal mRNA is made mostly from the P2 promoter.
At high cAMP-CRP concentrations, gal mRNA is made principally the P1 promoter.
Galactose operon
gal R
O
AS
P
O gal E gal T
gal K
Transferase
Epimerase
Gal
Gal-1-P
galK
galT
gal M
Mutarotase
Kinase
UDP-Gal + Glu-1-P
galE
UDP-Glu
The significance of two Promoters in the gal operon
Gal
Gal-1-P
galK
galT
UDP-Gal + Glu-1-P
galE
UDP-Glu
•
Galactose not only serve as a carbon source when it is all that is available but the
related compound uridinediphosphogalactose (UDPGal) is a precursor in the
synthesis of E. coli cell wall.
•
In the absence of exogeneous galactose, UDPGal is formed from UDP-glucose in a
reaction catalyzed by galactose epimerase, the product of the galE gene.
Arabinose Operon
Positive and Negative control by the same protein, Ara C
Arabinose metabolism
• The arabinose operon in E.coli, which encodes enzymes involved in the
utilization of the sugar arabinose, provides another example of variation in
the behavior of regulatory protein.
• The ara operon, like lac and gal is inducible, and not normally expressed.
• The Ara enzymes are made only when the cells are exposed to arabinose.
• The expression is regulated by a protein called AraC, which by itself acts
as a repressor.(Autogenous or auto-regulation)
• When arabinose is present, the sugar binds to AraC and convert it into an
activator.
• The arabinose-AraC complex activates transcription from the ara promoter.
Arabinose operon
araC P
O1
AS
P
araB
araA
araD
• The ara operon codes for three enzymes that are required to catalyze
the metabolism of arabinose.
• Arabinose isomerase - encoded by araA - coverts arabinose to ribulose
• Ribulokinase - encoded by araB - phosphorylates ribulose
• Ribulose-5-phosphate epimerase - encoded by araD -- converts ribulose5-phosphate to xylulose-5-phosphate which can then be metabolized via
the pentose phosphate pathway.
Tryptophan Operon
Trp operon - gene activity is repressed when a chemical is added (tryptophan) unlike
Lac operon which is induced when chemical is added (allolactose) structural genes
(trpA, trpB, trpC, trpD, trpE)
Tryptophan operon(trp operon)
Structural Genes
TrpP,
O
Low Trp
High Trp
trpL
trpE
trpD
trpC
trpB
trpA
trp operon
Leader peptides in operon subject to attenuation
참고문헌 및 web sites
1. Molecular Biology-Genes to Proteins,(4th ed.), BE Tropp, Jones & Bartlett(2012)
2. GENES X, Krebs et al., Jones & Bartlett(2011)
3. Molecular Biology(4th ed.), RF Weaver, McGraw Hill(2008)
4. Molecular Biology of the gene(6th ed.), Watson et al, CSH Press(2008)
5. Essential GENES(2nd ed.), Krebs et al ., Jones & Bartlett(2010)
6. 분자생물학 입문, 대표역자 송민동 , ㈜월드사이언스(2011)
N•E•X•T
• Translation