Trp repressor

The trp (tryptophan) repressor is a 25 kD protein which regulates transcription of the tryptophan biosynthetic pathway. There are 5 operons which are regulated by trpR: the trpEDCBA, trpR, AroH, AroLand mtr operons.

When the amino acid tryptophan is in plentiful supply in the cell, trpR binds 2 molecules of tryptophan, which alters its structure and dynamics so that it becomes able to bind to operator DNA. When this occurs, transcription of the DNA is prevented, suppressing the products of the gene – proteins which make more tryptophan. When the cellular levels of tryptophan decline, the tryptophan molecules on the repressor fall off, allowing the repressor to return to its inactive form.

trpR also controls the regulation of its own production, through regulation of the trpR gene (Kelley & Yanovsky, 1982 PNAS USA 79 3120–3124).

The structure of the ligand-bound holorepressor, and the ligand-free forms have been determined by both X-Ray crystallography and NMR (Schevitz et al, 1985 Nature 317 782–786, Otwinowski et al., 1988 Nature 335 321–329,Lawson & Carey, 1993 Nature 366 178–182,Zhao et al., 1993 J Mol Biol 229 735–746,Zhang et al., 1994 J Mol Biol 238 592–614).