Gene regulation in prokaryotes/Trp operon

The Repressible Tryptophan Operon

trpR

The trp repressor gene, trpR, is not closely linked to the trp operon and trpR protein is always produced. In the presence of tryptophan, trpR repressor protein undergoes a conformational changes and binds to the operator, preventing transcription.

trpL and Attenuation

The trpL region lies 5' to the genes that encode tryptophan synthetic enzymes. Attenuation results from premature termination of trp RNA at the attenuator sequence in trpL and requires tryptophan-charged tRNA^Trp. RNA transcribed from the attenuator sequence forms a hairpin.

The 162 nucleotide trp leader contains four regions that can base pair with itself and for alternative stem-loop structurs.

The leader sequence contains a short open reading frame coding for a 14 amino acid peptide followed by a stop codon. This peptide includes two consecutive trp codons. The production of this peptide is modulated by the availability of charged tRNA^Trp