Lac and Trp Operon - Lecture 11 Flashcards
What is an operon?
Cluster of genes transcribed by the same promoter that gives rise to a polycistronic mRNA
What does the lacI gene code for?
Lac repressor protein
How does the lac repressor protein work?
○ Binds to Olac (operator)
○ Holds RNA polymerase in place
○ Prevents transcription of lacZ, lacY, lacA
What does lacZ code for?
β-galactosidase
What does lacY code for?
Permease
What does lacA code for?
Acetylase
How is the lac operon switched on?
○ Lactose binds to repressor protein and forces it to dissociate from the operator
○ Polymerase can now transcribe the gene
○ Under negative feedback control
What are cis-acting elements?
○ Will only regulate DNA to which it is directly joined to
○ “Dominant” - any mutation cannot be undone
What are trans-acting factors?
○ Made somewhere else by a trans-acting gene
○ Will regulate genes anywhere
○Mostly protein transcription factors
○ Recessive - defective protein can be restored/swapped for a functional protein
What happens if the operator is mutated in the lac operon?
○ Repressor can’t bind so gene is always switched on
○ Constitutive
What happens if the repressor is mutated in the lac operon?
○ Mutated repressor can’t bind to operator so gene is always switched on
○ Constitutive
What happens if a new repressor gene was introduced to the lac operon with a mutated operator?
○ No effect
○ Repressor still can’t bind
○ Cis-dominant
What happens if a new repressor gene is introduced to the lac operon with a mutated repressor gene?
○ Complements
○ New repressor can bind to promoter
○ Trans-recessive
What is catabolite repression?
Bacteria won’t metabolise lactose if there is glucose present
What does the lag show in catabolite repression graph?
Time to activate operon
How does glucose repress other operons?
○ RNA polymerase requires CRP
○ cAMP binds to CRP permitting DNA binding
○ High glucose: low cAMP so lac operon if off
○ Low glucose: high cAMP so lac operon is on if lactose is present
What does the Trp operon contain?
Genes for synthesis of amino acid tryptophan
How is tryptophan regulated?
○ Trp repressor cannot bind without tryptophan
○ Enzymes for tryptophan synthesis are made
○ Tryptophan made and makes trp repressor active
○ Active trp repressor binds to operator which prevents tryptophan synthesis
What do products and subtrates do?
○ Products of biosynthetic operon repress their operons (tryptophan)
○ Substrates for metabolism induce their operons (lactose)
