The structure of Rph , an exoribonuclease from Bacillus anthracis , at 1 . 7 Å

Maturation of tRNA precursors into functional tRNA molecules requires trimming of the primary transcript at both the 5’ and 3’ ends. Cleavage of nucleotides from the 3’ stem of tRNA precursors, releasing nucleotide diphosphates, is accomplished in Bacillus by a phosphate dependent exoribonuclease, Rph. The crystal structure of this enzyme from Bacillus anthracis has been solved by molecular replacement to a resolution of 1.7 Å and refined to an R factor of 19.3%. There is one molecule in the asymmetric unit; the crystal packing reveals the assembly of the protein into a hexamer arranged as a trimer of dimers. The structure shows two sulphate ions bound in the active site pocket, probably mimicking the phosphate substrate and the phosphate of the 3’ terminal nucleotide of the tRNA precursor. Three other bound sulphate ions point to likely RNA binding sites.