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Pat O’Farrell Dept. Biochem and Biophys. UCSF San Francisco. Characterizing expression differences. Characterizing expression differences. S u p e r c e d e d. . Looking for ONE difference (needle in the hay stack). Looking for ONE difference (needle in the hay stack). . - PowerPoint PPT Presentation
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Pat O’FarrellPat O’FarrellDept. Biochem and Biophys.Dept. Biochem and Biophys.UCSFUCSFSan FranciscoSan Francisco
Characterizing expression differencesLooking for ONE difference (needle in the hay stack)Assaying ONE differenceDefining a complex (pleiotropic) response
S u p e r c e d e d
2D Gels/proteomics - what is it good forArrays Arrays
ArraysArr
ays ArraysArrays
ArraysArrays
Arrays
Characterizing expression differencesLooking for ONE difference (needle in the hay stack)Assaying ONE differenceDefining a complex (pleiotropic) response
Characterizing expression differencesLooking for ONE difference (needle in the hay stack)Assaying ONE differenceDefining a complex (pleiotropic) response
2D Gels/proteomics - what is it good for
Characterizing expression differencesLooking for ONE difference (needle in the hay stack)Assaying ONE difference
Defining a complex (pleiotropic) response?
Assessing & comparing protein levels/tissues/secretions
Regulation of protein levels by turnover
Assessing modification of proteins
Finding surprisesFinding surprises
Defining a complex (pleiotropic) responseDefining a complex (pleiotropic) response
The Lac operon and catabolite repression
Glucose Genes for catabolism of other metabolites
Cyclic AMPGlucose Genes for catabolism of other metabolites
CrpCrpcAMP
cAMP cAM
PcAM
P
P Lac
-galactosidase
100
50
25
100
50
25
Control
+ cy AMPRepression by cy AMP
Control+ cy AMPControl+ cy AMPControl+ cy AMPControl+ cy AMPControl+ cy AMPControl+ cy AMPControl+ cy AMPControl
+ cy AMP
The catabolite repression (cyAMP) domain
Size: -about 10% of all genes respond to cyclic AMP
Heterogeneity: -responses vary - in direction (~1% repression 9% induction) - and magnitude
Mechanism: - all responses depend on the same receptor • i.e. crp mutants show no response to cyclic AMP - the receptor is inactive without cyclic AMP • i.e. adenyl cyclase mutants = crp mutants
Overlap - cyclic AMP responsive genes are downregulated by limitation for an amino acid
Warning!
• strains must be congenic
The analysis is EXTREMELY sensitive to conditions
• culture conditions must be precisely reproduced
• Synthesis of ribosomes is a major part of bacterial growth• Ribosomes are half RNA and half protein
• aa starvation diminishes synthesis of ribosomal proteins• Stringent E. coli strains rRNA synthesis when short of aa• Relaxed strains lack the ability to down regulate rRNA
rel+coordinate rRNA & r-proteinsynthesisrel-lack coordination ofrRNA & r-proteinsynthesis
Coordinating growth
G tetraphos&
G pentaphosSignals for
aa starvationrel
ATP + GTP
AMPAMP + + ppppGpppGppp
ppppGppGpp
GDPspoT
A system that detects shortage of aa & signals starvation
rRNArRNA synthesissynthesis
Experiment to test effect of ppGpp on protein expression• ppGpp does not get into cells I could not just add it a test the consequences
Compare+ppGpp -ppGpp
rel+ strain
Restrict amino acids
Residual protein synthesis occurs in the presence of ppGpp
rel- strain
Restrict amino acids
Residual protein synthesis occurs in the absence of ppGpp
• My Plan
Contro
l
Residual protein synthesis in starved rel+ or rel -
35S-methionine incorporation
10 to 20% residual incorporation during starvation
-R induced proteins
Specific responses to particular -aa
-R -H -P -Lrel+
Contro
l
-R -H -P -L-R -H -P -Lrel+ rel-
Residual protein synthesis in starved rel+ or rel -
Specific responses to particular -aa
Global changes with in MW
The “Hungry Codon”
• a 20 residue protein with each of the aa • a step time, ST, is the time it normally takes to add one aa• its synthesis would take 20 ST
• simple thought experiment
C+1
D+1
E+1
F+1
G+1
H+1
I+1
K+1
L+1
M+1
N+1
P+1
Q+1
R+1
S+1
T+1
V+1
W+1
Y+1
A1
• starvation for histidine reduces protein synthesis to 10%
The “Hungry Codon”
• simple thought experiment
C+1
D+1
E+1
F+1
G+1
H+1
I+1
K+1
L+1
M+1
N+1
P+1
Q+1
R+1
S+1
T+1
V+1
W+1
Y+1
A1
• starvation for histidine reduces protein synthesis to 10%• rate of synthesis of average peptide is reduced 10x
H+181
I+1
K+1
L+1
M+1
N+1
P+1
Q+1
R+1
S+1
T+1
V+1
W+1
Y+1
C+1
D+1
E+1
F+1
G+1
A1
• it takes 10x as long to synthesis the average peptide• it takes 10x20=200 ST to make our 20 aa peptide• 19 of aa are normal and are added in 19 step times• translation of the hungry codon takes 200-19=181 ST
• a 20 residue protein with each of the aa • a step time, ST, is the time it normally takes to add one aa• its synthesis would take 20 ST
A
B
C
D
F
G
A steady state determines pool size
Sucking the pool dry (almost)
-histidine -proline
In rel- cells, aa starvation induces errors in translation
• during H starvation newly synthesized proteins are heterogeneous• interpretation: - an uncharged aa is occasionally incorporated in place of H - each substituted position removes a basic residue - the trail of spots is consistent with 3% misincorporation • starvation for different amino acids give different types of errors• interpretation: - termination occurs if an codon is not easily misread as another residue - charge errors occur if codon is easily misread as a differently charged residue
• •••• • •
Errors in translation are not seen in starved rel+ cells
Control -H rel+ -H rel-
How can ppGpp the fidelity of translation during starvation?
• ribosomal proteins L7 has no histidine
Starved rel+ cells behave as if they are not missing an aa
RelativeL7 expression
control +++ rel- - H +++++ rel- - I +
rel+ - H+++
rel+ - I+++
Difference consistent with the difference in H and I abundance in the protein
Expression insensitive to aa abundance
rel+ unstableppGpp
r R
N A
r R
N A
P r
o t e
i n
ppGpp Inhibits Protein Synthesis as well as rRNA Synthesis
rel- no ppGpp
rel+ spoT-
stableppGpp
Control - aa - + aa
Sharing the burden
• in a rel- strain all the slowing of translation occurs at the “hungry codon”
C+10
D+10
E+10
F+10
G+10
H+10
I+10
K+10
L+10
M+10
N+10
P+10
Q+10
R+10
S+10
T+10
V+10
W+10
Y+10
A10
H+181
I+1
K+1
L+1
M+1
N+1
P+1
Q+1
R+1
S+1
T+1
V+1
W+1
Y+1
C+1
D+1
E+1
F+1
G+1
A1
• ppGpp slows down translation at multiple steps of translation
• The aa-tRNA for the hungry codon is only reduced enough to generate a signal
ConclusionsConclusions
1. Protein synthesis has a precarious relationship with its substrates1. Protein synthesis has a precarious relationship with its substrates• imbalances in substrates are exaggerated as residues are incorporated according to the dictates of the code not availability
2. Substrate imbalance severely compromises fidelity2. Substrate imbalance severely compromises fidelity
• 3% missincorporation, tuncation & inactive enzymes
3. ppGpp makes translation more robust and accurrate3. ppGpp makes translation more robust and accurrate
• it adjusts protein synthesis rates to availability of the limiting aa (the weakest link)
• it acts as governor to coordinate translation with substrate supply
Generalization
• balanced substrate supply is universally important for translation
• a specific signaling system that senses substrate levels and modulates translation accordingly
Global requirement
The basis of this regulation outside of E coli is not known.It is not based on ppGpp, which is absent in eukaryotes.
rel+ spoT+
rel+ spoT-
rel-
- aa (I) +I (0-5 min) +I (12.5 min)ppGpp
t
t
t
[C]
[C]
[C]
ppGpp -aa
Specific responses to ppGpp
rel+ spoT+
rel+ spoT-
rel-
- aa (I) +I (0-5 min) +I (12.5 min)ppGpp
t
t
t
[C]
[C]
[C]
ppGpp -aa
Specific responses to ppGpp
- aa (I) +I (0-5 min) +I (12.5 min)
ppGpp -aa
Specific responses to ppGpp
rel-
t
[C]
rel+ spoT-
t
[C]
rel+ spoT+
ppGpp
t
[C]
Pat O’FarrellDept. Biochem and Biophys.UCSFSan Francisco