Complexity in Regulation of Tryptophan Biosynthesis in Bacillus subtilis

Bacillus subtilis uses novel regulatory mechanisms in controlling expression of its genes of tryptophan synthesis and transport. These mechanisms respond to changes in the intracellular concentrations of free tryptophan and uncharged tRNA^Trp. The major B. subtilis protein that regulates tryptophan biosynthesis is the tryptophan-activated RNA-binding attenuation protein, TRAP. TRAP is a ring-shaped molecule composed of 11 identical subunits. Active TRAP binds to unique RNA segments containing multiple trinucleotide (NAG) repeats. Binding regulates both transcription termination and translation in the trp operon, and translation of other coding regions relevant to tryptophan metabolism. When there is a deficiency of charged tRNA^Trp, B. subtilis forms an anti-TRAP protein, AT. AT antagonizes TRAP function, thereby increasing expression of all the genes regulated by TRAP. Thus B. subtilis and Escherichia coli respond to identical regulatory signals, tryptophan and uncharged tRNA^Trp, yet they employ different mechanisms in regulating trp gene expression.