Tryptophan repressor
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Tryptophan repressor is a dimeric protein (2 x 12,500 Mr) which regulates transcription of at least 5 genes; When the amino acid tryptophan is plentiful in the cell, it binds to the protein, which then binds in turn to these 5 DNA sequences, shutting off the genes.
One of these genes, trpR, codes for the production of tryptophan itself. Another codes for the tryptophan repressor protein. These are forms of feedback regulation.