trp operon
The trp operon is a group of genes that are transcribed together, encoding the enzymes that produce the amino acid tryptophan in bacteria.
The trp operon contains five structural genes: trpE, trpD, trpC, trpB, and trpA, which encode the enzymes needed to synthesize tryptophan.
When tryptophan is present, the trpR protein binds to the operator, blocking transcription of the trp operon by RNA polymerase.
The repressor for the trp operon is produced upstream by the trpR gene, which is constitutively expressed at a low level.
[3] Attenuation is made possible by the fact that in prokaryotes (which have no nucleus), the ribosomes begin translating the mRNA while RNA polymerase is still transcribing the DNA sequence.
At the beginning of the transcribed genes of the trp operon is a sequence of at least 130 nucleotides termed the leader transcript (trpL; P0AD92).
[2][10][11][12] If the ribosome attempts to translate this peptide while tryptophan levels in the cell are low, it will stall at either of the two trp codons.
Mutational analysis and studies involving complementary oligonucleotides demonstrate that the stability of the 2–3 structure corresponds to the operon expression level.
[2][10] The end result is that the operon will be transcribed only when tryptophan is unavailable for the ribosome, while the trpL transcript is constitutively expressed.
[10][13] In contrast, complementary oligonucleotides targeting strand 1 increases the operon expression by promoting the antiterminator formation.
[10][14] Furthermore, in histidine operon, compensatory mutation shows that the pairing ability of strands 2–3 matters more than their primary sequence in inhibiting attenuation.
[2] To ensure that the ribosome binds and begins translation of the leader transcript immediately following its synthesis, a pause site exists in the trpL sequence.
[16] There is a 7th gene in Bacillus subtilis's operon called trpG or pabA which is responsible for protein synthesis of tryptophan and folate.
[18] In addition, the activated TRAP inhibits the initiation of translation of trpP, trpE, trpG and ycbK genes.
