Interaction of the 11-subunit trp RNA-binding attenuation protein (TRAP) with its RNA target.

The trp RNA-binding attenuation protein (TRAP) regulates expression of the B. subtilis trp operon by binding to an RNA target composed of 11 G/UAG repeats in the trp leader transcript. TRAP contains 11 identical subunits arranged in a ring. The binding of 11 molecules of L-tryptophan to sites formed between adjacent subunits activates TRAP to bind trp leader RNA. We propose that the trp leader RNA forms a matching circle on the surface of TRAP when it binds. RNA binding is relatively insensitive to changes in ionic strength and pH, and is apparently driven by changes in entropy. Two lysine residues on the surface of TRAP have been shown to be important for RNA binding.