Arginyl-tRNA Synthetase, a class I aminoacyl tRNA synthetase playing a crucial role in protein biosynthesis, has been crystallized for the first time. Polyethylene glycol (PEG) was used as a precipitant, and the crystallization proceeded at pH 6.5. These single crystals diffracted to 2.8 8, with a rotating anode X-ray source and R-axis IIc image plate detector. They have an orthorhombic space group P2,2,2 with unit cell parameters of a = 251.51 8,. b = 53.12 8,, and c = 52.35 8,. A complete native data set has been collected at 3.1 A resolution for these crystals.Keywords: arginyl-tRNA synthetase; class I aminoacyl-tRNA synthetase; crystallization; protein biosynthesis; X-ray crystallography Aminoacyl-tRNA synthetases play a crucial role in protein biosynthesis. Arginyl-tRNA synthetase (ArgRS) is classified as a member of the class I aminoacyl-tRNA synthetases (aaRS) with specificity towards the 2'-OH of the tRNA terminal adenosine, based on the available structure-function information for aaRSs (Eriani et al., 1990a, Nureki et al., 1995. ArgRS has also been well known as a representative enzyme in a small group of aaRS with glutamyl-and glutaminyl-tRNA synthetases. In this small group the presence of tRNA is necessary for the ATP-PPi exchange reaction, in contrast to the majority of aminoacyl-tRNA synthetases (Ravel et al., 1964(Ravel et al., , 1965 Parfait & Grojean, 1972;Gangloff et al., 1976;Char & Gopinathan, 1986;Lin et al., 1988a Lin et al., , 1988b. The ATP-PPI exchange is usually an easy measurement of the amino acid activation for aaRSs.As the above three enzymes are also classified in the same subclass (IC) of aaRS with charged or polar amino acids as the substrate, structural information will permit a very interesting deReprints request to: Dr. s.-X. Lin tailed comparison of their structure-function relationships. The three-dimensional structures of glutaminyl-and glutamyl-tRNA synthetases have been determined with enzymes from E. coli and 7: thermophilus, respectively. Although ArgRS from E. coli. has a sequence of HVGH associated with a Rossmann fold (Eriani et al., 1990a) similar to HIGH for most of the class I aminoacyl-tRNA synthetases, the enzyme does not have sequence homologies with other aminoacyl-tRNA synthetases. Thus, another interesting characteristic relevant to structure-function study of ArgRS will be provided by its crystallization and subsequent structure determination. Here we report the crystallization and the preliminary crystallographic study of ArgRS.