Trp operon Flashcards
Tryptophan operon: repressible operon
Codes for enzymes which synthesize amino acid tryptophan
It is regulated by repression and attenuation
Repression
Controls the initiation of transcription
Attenuation
Governs the frequency of premature transcript termination
Repressor protein
Can be activated by allosteric transition
TrpEDCBA genes
5 structural genes that code for enzymes that convert chorismic acid to tryptophan
Transcribed as a polycistronic mRNA that directs translation of enzymes to catalyze the biosynthesis of tryptophan
Pathway of chorismic acid to tryptophan
CA -> anthranilate -> phosphoribosyl anthranilate -> indole-3-glycerol phosphate -> tryptophan
trpP
Promoter region
trpO
Operator region
trpR gene
Located away from the tryptophan structural genes
Codes for trp operator protein
repression
controls the initiation of transcription
When tryptophan is absent
Trp operon is activated.
trpR is a repressor protein
Trp repressor is an inactive protein which is unable to bind to trp operon on its own
RNA polymerase bind to promoter region and transcribes the trpEDCBA structural genes to produce polycistronic mRNA
The trpEDCBA polycistronic mRNA is then translated by the ribosomes into the enzymes that synthesize the amino acid tryptophan.
When tryptophan is present
trp operon is inactivated
tryptophan acts as co-repressor and binds to inactive trpR repressor protein, cause an allosteric change to repressor protein
This tryptophan-repressor complex is now able to bind to the operator region, repressing the transcription of the 5 structural genes by blocking access to RNA polymerase.
Thus when tryptophan is present in excess, the tryptophan operon is repressed and the enzymes necessary for tryptophan synthesis are not
made.
Attenuation
A regulatory mechanism to prematurely terminate transcription of the mRNA
Another mechanism by which tryptophan inhibits enzyme synthesis.
The activated repressor (i.e. TrpR + tryptophan), even when bound to the operator region, does not strongly inhibit expression of the trp operon
Leader sequence in the trp operon
- > mRNA transcription is initiated by RNA polymerase within the trpP region and proceeds along a leader sequence.
- > Within this leader sequence, there are regulatory regions which is an integral part of the control mechanism of the trp operon.
-> When transcribed by RNA polymerase, the leader mRNA sequence can folds back on itself and form a stem loop (hairpin) via complementary
base-pairing
Leader sequence in trp operon
- > ribosomal binding site
- > a mini open reading frame
The mini ORF in leader sequence
- > AUG translation start codon
- > 2 trp (UGG) codons responsible for attenuation
- > UGA translation stop codon
Molecular explanation for attenuation
- Translation of the trpL gene produces a short polypeptide.
- Near the stop codon are two tryptophan (UGG) codons.
- Within the leader mRNA sequence are four regions that can form secondary structures by complementary base-pairing.
- Tight coupling of transcription and translation in prokaryotes makes control by attenuation possible.
- During the pause, a ribosome loads onto the mRNA and begins translation of the leader peptide.
- Ribosome position is key to attenuation.
- Secondary structure of the leader mRNA sequence leads to either continued transcription or termination (attenuation) by the RNA polymerase.
2 important stem loop hairpin structures:
Hairpin 3+4 -> termination hairpin (in presence of tryptophan)
Hairpin 2+3 -> anti-terminator (in absence of tryptophan)
When tryptophan concentration is high
Transcription of the leader sequence proceeds but premature termination (attenuation) of the mRNA synthesis occurs. Incomplete trp operon is transcibed
- > Then charged tRNAtrp is also present.
- > The ribosome successfully translate the leader sequence, ending in region 2.
- > This prevents region 2 from pairing with region 3, leaving region 3 available to pair with region 4.
-> Pairing of regions 3 and 4 creates a rho-independent terminator known as the
attenuator.
-> The hairpin formed by regions 3+4 is a signal for RNA polymerase to terminate transcription before the structural genes are reached.
When tryptophan is absent or low in concentration
The inactive Trp repressor cannot bind to the operator and attenuation is bypassed. Entire trp operon is transcribed by RNA polymerase.
- > Then charged tRNAtrp are unavailable.
- > The ribosome stalls during the translation of the trp (UGG) codons in the leader sequence, covering region 1.
- > When region 1 is covered by the ribosome, it cannot pair with region 2.
- > Region 2 pairs with region 3 instead.
- > The hairpin formed by regions 2+3 is a signal for RNA polymerase to continue mRNA transcription of the trpEDCBA structural genes.
- > Once region 3 is paired with region 2, it is unable to pair with region 4.
- > Without the hairpin formed by regions 3+4, attenuation is bypassed and transcription continues through the structural genes.
Conclusion for trp operon
Thus, mRNA transcription may still occur when the Trp
repressor is bound to the operator, but transcription is
terminated in the presence of high concentration of
tryptophan through the attenuation mechanism
