Prokaryotic gene expression Flashcards
What is an operon?
Set of genes that are regulated by a single promoter–1 mRNA, multiple proteins
What is an operator?
Regulatory sequence bound by a specific regulator or activator
What genes are included on the lac operon?
Beta-galactosidase : hydrolyzes lactose
lactose permease : promotes lactose transport in to cells
thiogalactoside transacetylase : function unknown
What is constitutive expression?
Expression regardless of conditions (as opposed to inducible expression)
What is LacI? What does it do?
Repressor for Lac operon, encoded right next to operator
If Lactose is absent-
Repressor protein binds to operator, which is right next to promoter–when this happens, polymerase cannot bind, no gene expression
If lactose is present-
Modified lactose (allolactose) binds to LacI, preventing it from binding to the operator/causing it to fall off
Genes in the operon are expressed
What is CAP? How does the lac operon sense glucose?
Sensed indirectly by levels of cAMP
CAP = catabolite gene activator protein, binds upstream of lac when glucose is low
CAP senses glucose indirectly
- cAMP levels are inversely correlated with glucose levels
CAP binds cAMP, and can then bind lac promoter, promoting transcription
What is attenuation?
Expression regulated by availability of certain amino acids
Regulation after transcription initiation
What is the riboswitch?
Regulation by binding small metabolite
What is the common feature between attenuation and riboswitch?
Changes in structure of RNA at the 5’ untranslated region–can form 2 mutually exclusive structure–regulates expression
What is the structure of trp operon?
5 genes which encode 3 enzymes
Operator and repressor (trpR)
How does trpR work?
Must bind to tryptophan before it can bind to operator
binding of trpR turns off tryptophan synthesis
How is trp regulated by attenuation?
2 RNA sequences possible at 5’ UTR:
1) intrinsic termination structure–stable hairpin followed by string of U’s
2) Alternative structure that blocks formation of intrinsic termination structure
How is 5’UTR RNA structure regulated?
Translation of leader peptide upstream of start codon of first trp gene regulates RNA structure
How does the leader peptide regulate RNA structure?
Leader peptide has 2 adjacent tryptophans
(remember, in prokaryotes, translation begins as soon as head of the mRNA begins to emerge from polymerase)
if high tryptophan, translation is fast
if low tryptophan, translation is slow
What are riboswitches?
Untranslated mRNA w/ secondary structure, binds small metabolites and regulates expression
