The Tryptophan Repressor

The genes encoding these are clustered together in a single operon with its own promoter and operator. When tryptophan is available to the cell, its presence shuts down the operon.

• One molecule of tryptophan binds to a site on each monomer of the Trp repressor.
• The Trp repressor, a homodimer of two of these complexes, binds to the operator of the Trp operon.
• This shuts down transcription of the 5 genes of the operon so the enzymes used in Trp synthesis are not synthesized.

When tryptophan is not present in the cell, the repressor leaves the operator, and transcription of the 5 genes for tryptophan synthesis can begin.

This stereoscopic view (courtesy of P. B. Sigler) shows the tryptophan repressor (right side of each panel) bound to its operator DNA (left side). The two identical polypeptides of the repressor are shown on either side of the horizontal red line. The two tryptophan molecules are shown as red rings. (Look also for the stretches of alpha helix in each monomer.)

You may find it easier to fuse the two images into a 3D view by holding a sheet of 8.5 x 11" (22 x 28 cm) paper vertically between your nose and the dividing line between the two images on the screen so that your left eye sees only the left image, your right eye only the right. Persevere: the results are worth it!