Everything about Trp Operon totally explained
Trp operon is an anabolic
operon which promotes the production of
tryptophan in the absence of tryptophan in the environment. This is regulated both by repression and by attenuation.
Repression
The
repressor for the trp
operon is produced upstream by the trpR gene, which is continually expressed. It creates monomers, which associate into
tetramers. When tryptophan is present, it binds to the
tryptophan repressor tetramers, and causes a change in conformation, which allows the
repressor to bind the
operator, which prevents
RNA polymerase from binding or transcribing the operon, so
tryptophan isn't produced. When
tryptophan isn't present, the
repressor can't bind the operator, so transcription can occur. This is therefore a negative feedback mechanism.
Attenuation
Because repression of this operon is still "leaky," another system of controlling expression is also needed:
Attenuation. At the beginning of the transcribed genes of the trp operon is a leader sequence, which codes for a very short
polypeptide. Near the end of this sequence, two tryptophans are coded for next to each other. Because
tryptophan is a fairly uncommon amino acid, this is highly unusual. Since in
prokaryotes the ribosomes begin translating the
mRNA as soon as the
RNA polymerase has moved farther down the DNA sequence, upstream
translation occurs simultaneously with
transcription of downstream genes. So, as soon as the polymerase has created the mRNA for the leader sequence, it's being translated. When the ribosome reaches the double-trp codons, if enough trp is present, the ribosome won't be delayed, and will continue translating until it reaches the stop codon and falls off the leader transcript. A
hairpin will then form in the mRNA transcript (remember, still attached to RNA polymerase on other end) between regions 1-2, and 3-4, which destabilizes the RNA polymerase and halts transcription of the rest of the operon, thus preventing production of trp. On the other hand, if there's little or no trp available, the ribosome will be delayed or stopped on the double-trp, and a hairpin will form between regions 2-3 of the mRNA instead. This doesn't destabilize the polymerase, so transcription and translation occur.
Similar mechanism regulates the synthesis of
histidine,
phenylalanine and
threonine.
Further Information
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