Regulatory features of the trp operon and the crystal structure of the trpRNA-binding attenuation protein from Bacillus stearothermophilus

Characterization of both the cis and trails-acting regulatory elements indi cates that the Bacillus stearothermophilus trp operon is regulated by an at tenuation mechanism similar to that which controls the trp operon in Bacill us subtilis. Secondary structure predictions indicate that the leader regio n of the try mRNA is capable of folding into terminator and antiterminator RNA structures. B. stearothermophilus also encodes an RNA-binding protein w ith 77% sequence identity with the RNA-binding protein (TRAP) that regulate s attenuation in B. subtilis. The X-ray structure of this protein has been determined in complex with L-tryptophan at 2.5 Angstrom resolution. Like th e B. subtilis protein, B. stearothermophilus TRAP has 11 subunits arranged in a ring-like structure. The central cavities in these two structures have different sizes and opposite charge distributions, and packing within the B. stearothermophilus TRAP crystal form does not generate the head-to-head dimers seen in the B. subtilis protein, suggesting that neither of these pr operties is functionally important. However, the mode of L-tryptophan bindi ng and the proposed RNA binding surfaces are similar, indicating that both proteins are activated by L-tryptophan and bind RNA in essentially the same way. As expected, the TRAP:RNA complex from B, stearothermophilus is signi ficantly more thermostable than that from B, subtilis, with optimal binding occurring at 70 degrees C. (C) 1999 Academic Press.