Interaction of the trp RNA-binding attenuation protein (TRAP) with anti-TRAP

The trp RNA-binding attenuation protein (TRAP) negatively regulates expression of the tryptophan biosynthesis genes of Bacillus subtilis. In the presence of tryptophan, TRAP is activated to bind to the 5′-leader region of the trp mRNA resulting in termination prior to the structural genes. In addition, accumulation of uncharged tRNA^Trp induces synthesis of anti-TRAP (AT), which binds to TRAP and inhibits its function. Both of these proteins consist of oligomers of identical subunits. Here, we characterize the self-association of each of these proteins and the TRAP-AT interaction in free solution using equilibrium and velocity analytical ultracentrifugation. TRAP exists as a stable 11-mer in the absence and in the presence of tryptophan. Tryptophan binding induces a conformational change in TRAP. AT exists in a reversible equilibrium between trimer and dodecamer with an equilibrium constant of ∼3×10¹⁴M^-3. About 20% of the trimer is incompetent to form dodecamer. The AT equilibrium is slow on the time-scale of the velocity experiment. Formation of TRAP-AT complexes occurs only in the presence of tryptophan. A complex containing one TRAP 11-mer and one AT 12-mer forms with high affinity. At higher ratios of TRAP:AT complexes containing two TRAP 11-mers and one AT 12-mer are detected. A model for the structure of the complex is proposed.