Control of Prokaryotic Gene Expression - Operons Flashcards
Define: Operon
Cluster of genes with related functions; consisting of an operator, a common promoter and 2 or more structural genes, controlled as a single unit to produce a single polycistronic mRNA
Negative control
Suggests that regulatory gene of operon encodes for a REPRESSOR
Positive control
Suggests that regulatory gene encodes for an ACTIVATOR
Inducible system
- Suggests that default state of operon is OFF, but can be turned on in the presence of an INDUCER
Repressible system
- Suggests that default state of operon is ON, but can be turned off in the presence of a CO-REPRESSOR
Negative inducible control
- repressor normally bound to operator, preventing binding of RNA polymerase to promoter; no transcription of structural genes
- presence of inducer inactivates repressor by altering tertiary structure and making it unable to bind to operator; transcription of structural genes
Negative repressible control
- repressor unable to bind to operator in normal conformation, allowing RNA polymerase to bind to operator; transcription of structural genes
- presence of co-repressor activates repressor by altering tertiary structure and making it able to bind to operator; no transcription of structural genes
Positive inducible control
- activator unable to bind to operator in normal conformation, preventing binding of RNA polymerase to promoter; no transcription of structural genes
- presence of inducer activates activator by altering tertiary structure and making it able to bind to operator; transcription of structural genes
Positive repressible control
- activator normally bound to operator, allowing RNA polymerase to bind to promoter; transcription of structural genes
- presence of co-repressor inactivates activator by altering tertiary structure and making it unable to bind to operator; no transcription of structural genes
What kind of operon is the lac operon?
Negative inducible operon
Type of mechanism lac operon is associated with
Catabolism
Structural genes in lac operon
- lac Z
- lac Y
- lac A
What does lac Z encode for?
β-galactosidase
What does lac Y encode for?
Permease
What does lac A encode for?
Transacetylase
Function of β-galactosidase
- Hydrolyse lactose into glucose and galactose
2. Isomerise lactose into allolactose (inducer)
Function of Permease
Allows efficient uptake of lactose into E. coli cells by active transport
Function of Transacetylase
Transfer acetyl groups from acetyl CoA to β-galactosides to prevent build-up of toxic products formed by β-galactosidase
Promoter of structural genes of lac operon
Lac P
Operator of lac operon
Lac O
Regulatory gene of lac operon
Lac I
What does lac I encode for?
Active lac repressor
(-ve) Regulation of lac operon - absence of lactose
- regulatory gene, lac I, constitutively transcribed to produce lac repressor
- active lac repressor binds to operator, preventing RNA polymerase from binding to promoter, lac P; no transcription of structural genes and lac operon is OFF
(-ve) Regulation of lac operon - presence of lactose
- some lactose molecules enter E. coli cells aided by permease, before being converted to allolactose, an inducer, by β-galactosidase
- allocatose binds to repressor, inactivating it by altering tertiary structure and making it unable to bind to operator
- RNA polymerase binds to promoter; transcription of structural genes and lac operon is ON
(+ve) Regulation of lac operon - presence of lactose and glucose
- metabolise glucose preferentially; glucose imported cell, inactivating permease and preventing uptake of lactose
- genes involved in metabolising other sugars are repressed through catabolite repression
- glucose inhibits adenylyl cyclase and thus reduces conversion of ATP into cyclic adenine monophosphate (cAMP); fewer cAMP-CAP complexes formed
- inactive CAP unable to bind to CAP binding site; decreased binding efficiency of RNA polymerase and thus reduced transcription of structural genes
(+ve) Regulation of lac operon - presence of lactose, absence of glucose
- no inhibition of adenylyl cyclase by glucose and thus conversion of ATP into cyclic adenine monophosphate (cAMP); cAMP-CAP complexes formed
- active CAP binds to CAP binding site; increased binding efficiency of RNA polymerase and thus increased transcription of structural genes
Regulatory mechanism ensuring lac operon is OFF in presence of glucose
- Low affinity of RNA polymerase for promoter
2. Glucose-sensitive regulatory mechanism; CAP and CAP binding site
What kind of operon is the trp operon?
Negative repressible operon
Type of mechanism trp operon is associated with
Anabolism
Structural genes in trp operon
- trp E
- trp D
- trp C
- trp B
- trp A
Regulatory gene of trp operon
Trp R
What does trp R encode for?
Inactive trp repressor
Regulation of trp operon - high levels of tryptophan
- tryptophan, co-repressor, binds to inactive trp repressor, activating it by altering tertiary structure
- trp repressor binds to operator, preventing RNA polymerase from binding to promoter; no transcription of structural genes
Regulation of trp operon - low levels of tryptophan
- inactive trp repressor unable to bind to operator, RNA polymerase binds to promoter; transcription of structural genes
