REGULATION OF TRYPTOPHAN BIOSYNTHESIS - TRP-ING THE TRAP OR HOW BACILLUS-SUBTILIS REINVENTED THE WHEEL

The Bacillus subtilis tryptophan biosynthetic genes are regulated by T RAP, Radiographic crystallography indicates that the TRAP complex cont ains 11 identical subunits arranged in a doughnut-like structure terme d the beta-wheel, The trpEDCFBA operon is regulated by an attenuation mechanism in which tryptophan-activated TRAP binds to 11 (G/U)AG repea ts in the trp leader transcript, TRAP binding blocks formation of an a ntiterminator structure, thereby promoting the formation of an overlap ping terminator, resulting in transcription termination preceding the structural genes, When TRAP is not activated, it is unable to bind to the transcript, which allows anti-terminator formation and, hence, tra nscription of the operon, TRAP is also responsible for regulating tran slation of trpE and trpG. TRAP binding to trp operon readthrough trans cripts promotes refolding of the RNA such that the trpE Shine-Dalgarno sequence is sequestered in a hairpin, thus inhibiting TrpE synthesis, In the case of trpG, TRAP binds to nine repeats that overlap the ribo some-binding site, thereby blocking translation.