lecture 3 the regulation of gene expression in eukaryotes. s....
TRANSCRIPT
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Lecture 3
The regulation of gene expression in eukaryotes. S. cerevisiae Gal4 as a guide for universal principles of
transcriptional regulation
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•The Lac operon decides to use or not to use lactose
•The λ phage deciding whether to sail or to jump in an E. coli cell
•Galactose utilization in yeast
•Making stripes in the Drosophila embryo
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•Eukaryotes need to cater for the same as bacteria but also sexual reproduction, more complex and social environments
•Eukaryotes have a more complex structure more genes, MORE COMPUTATIONAL POWER
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http://es.wikipedia.org/wiki/Archivo:Budding_yeast_Lifecycle.png
Saccharomyces cerevisiae
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Glucose Galactose
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Gal1
Gal7
Gal2
Gal10
Gal5
Saccharomyces cerevisiae Galactose utilization
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Gal1Gal2Gal7Gal10
Gal4 Gal80Gal3 Gal11
There are genes that code for enzymes and genes that code for......something else. Some transcription factors but also other proteins
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Gal1Gal2Gal7Gal10
Gal4 Gal80Gal3 Gal11
Gal4 encodes a DNA binding protein
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GAL1++--
Deletion analysis uncovers a 400 bp region which is sufficient for GAL1 expression, and within it a small region that is essential
Reporters facilitates the analysis of gene regulation
Reporter
ß-galactosidase
Green Fluorescent ProteinGFP
GAL1
TATA
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275bp UAS GAL4
GAL1Mig1
@ 400bp
Within the 400bp required for expression, 275bp from the transcription start site there are four binding sites for Gal4 (UASGal4 for Upstream Activating Sequences for Gal4) which promote GAL1 expression
....there is also a binding site for a repressor, Mig1, which represses GAL1 expression in the presence of Glucose.
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Gal1Gal2Gal7Gal10
Gal4 Gal80Gal3 Gal11
Every gene encoding enzymes has a Gal4 binding site
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Gal1
Gal4 Gal80Gal3 Gal11
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From Ptashne and Gann, Genes and signals
Mig1 is a repressor which ensures that GAL genes are only used when there is no Glucose around. It works in a different manner from bacterial repressors as it acts by recruiting a number of proteins which make the promoter inaccesible to RNA polymerase
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From Ptashne and Gann, Genes and signals
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GAL1Mig1
GAL1Mig1
GAL1Mig1
Gal80
Gal3
GAL1Mig1
Gal4
Gal4 Gal80Gal3 Gal11
No galactose Galactose
OFF ON
Gal3 shuttles between the cytosol and the nucleus and, in the presence of Galactose, binds Gal80 releasing the activity of Gal4 and providing a rapid induction of transcriptional activity
Gal3
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GAL1
GAL1
Gal80
RNA polII
GAL1Mig1
Gal11MEDIATORGal4
Gal4 Gal80Gal3 Gal11
Gal11 is a component of the MEDIATOR complex which, through an interaction with Gal4, allows the link between Gal4 and the basal transcriptional machinery
Galactose
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From Ptashne and Gann, Genes and signals
!
λ repressor (cI)
Gal4 is a transcriptional regulator
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From Ptashne and Gann, Genes and signals!
!
λ repressor (cI)
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From Ptashne and Gann, Genes and signals
The Gal4 protein is structurally and functionally MODULAR
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From Ptashne and Gann, Genes and signals
The Gal4 protein is structurally and functionally MODULAR
LexA is a bacterial transcriptional regulator with a high affinity DNA binding site. Of course one has to provide a LexA binding site in yeast but this is no problem.
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From Ptashne and Gann, Genes and signals
The Gal4 protein is structurally and functionally MODULAR
LexA is a bacterial transcriptional regulator with a high affinity DNA binding site. Of course one has to provide a LexA binding site in yeast but this is no problem.
LexA binding site
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How specific is the AD of Gal4?
From Ptashne and Gann, Genes and signals
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The modular nature of transcriptional regulators
DNA binding domainGal4, LexA, cI....
Activation domainVP16, Gal4,.......
Repression domainenR, Mig1,.....
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DNA binding
Trans Activation
Transcription factors are modular
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cell type or tissue specific enhancer
Yeast Gal4 works in Drosophila
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lacZ
Fluorescent proteins
cell type or tissue specific enhancer
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....and in fish
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Summary
•The structure of an eukaryotic cells mirrors (suits) its computational capacity.
•Regulated recruitment and cooperativity are central elements of the transcriptional process that promotes functionality with a ‘small’ number of transcription factors.
•Transcription factors are modular (DNA binding + Activator or Repressor domains).
•The interactions between transcription factors and the basal transcriptional machinery are Universal.
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Drosophila development is about making segments, making stripes
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In bicoid (bcd) mutants, all anterior structures are missing
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Bcd
Bicoid protein
bicoid encodes a transcription factor that distributes itself in a gradient across the embryo (it can do this because the embryo is a syncitium at this early stage)
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http://scienceblogs.com/pharyngula/
The position of pattern landmarks is determined by the local concentration of Bicoid
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hunchback(hb)
Bcd
HD AD
Bicoid (bcd) No Bicoid (bcd)
Bicoid encodes a transcription factor and the hunchback (hb) gene is a target of Bicoid which responds at a certain concentration
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High a!nity Low a!nity
Bicoid gradient hunchback or syntheticbinding sites
reporter expression in embryo
The combination of Bicoid binding sites with different affinities in the promoter of hunchback, determines the pattern of response; the domain
of expression of the hunchback gene
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High a!nity Low a!nity
Bicoid gradient hunchback or syntheticbinding sites
reporter expression in embryo
Synthetic High a!nity
Synthetic Low a!nity
The combination of Bicoid binding sites with different affinities in the promoter of hunchback, determines the pattern of response; the domain
of expression of the hunchback gene
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Bicoid (bcd)
bcdhb bs GAL1 lacZ
HD AD
Gal4 AD
o
o
Bicoid has a DNA binding domain, homeodomain (HD) and an activation domain (AD) which can be substituted by the yeast Gal4 AD.
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A cascade of transcription factors
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Borok M J et al. Development 2010;137:5-13
Transcription factor gradients activate even skipped expression.
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Bcd
Hb Gt Kr
eve 2
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Bcd
Hb Gt Kr
eve 2
Bcd
Hb
eve 2
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In summary
•The spatial localization of domains of gene expression linked to pattern formation relies on the deployment of transcription factors in specific domains.
•The deployment is mediated by transcription factors or combinations of transcription factors (activators and repressors).
•The process relies on a combination of regulated recruitment and differential affinities.
•The programmes are time chains of transcriptional regulation.
•The transcriptional mechanism is universal.
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Bcd
Hb
eve 2
I lacZYA
lactose Y
lac Operon
Bcd
Hb Gt Kr
eve 2
cI cro cII O P
N
pL
pR
pRE
int
cI cro
LysisLysogeny
Positive autoregulatory loop Bistable switch Feedforward loop
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