Fig. 1.
Transcription and translation of the trp operon in B. subtilis is regulated by feedback loops involving the functions of two proteins: the ring-shaped 11-mer TRAP, which senses free Trp, and Anti-TRAP (AT) which is overexpressed in response to accumulation of uncharged tRNATrp. In the absence of AT, Trp-bound TRAP (blue) binds a series of UAG and GAG triplets in the 5′ leader of the trp operon, resulting in transcriptional and translational repression in part by formation of a terminator or sequestering the Shine-Dalgarno (SD) sequence. When expressed, trimeric AT (AT3) can bind to Trp-activated TRAP, preventing RNA binding and blocking its inhibition of Trp biosynthesis. AT exists in equilibrium between trimeric (red) and dodecameric states (blue, orange, green; the red trimer is behind the other three), while only trimeric AT binds and inhibits TRAP.