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The control of tryptophan production in Bacillus is a paradigmatic example of gene regulation involving the interplay of multiple protein and nucleic acid components. Central to this combinatorial mechanism are the homo-oligomeric proteins TRAP (trp RNA-binding attenuation protein) and anti-TRAP (AT). TRAP forms undecameric rings, and AT assembles into triskelion-shaped trimers. Upon activation by tryptophan, the outer circumference of the TRAP ring binds specifically to a series of tandem sequences present in the 5′ UTR of RNA transcripts encoding several tryptophan metabolism genes, leading to their silencing. AT, whose expression is up-regulated upon tryptophan depletion to concentrations not exceeding a ratio of one AT trimer per TRAP 11-mer, restores tryptophan production by binding activated TRAP and preventing RNA binding. How the smaller AT inhibitor prevents RNA binding at such low …