Department of Bioscience and Biotechnology, Tokyo Institute of Technology^* Department of Molecular Biophysics and Biochemistry, Yale University, USA^**
○Yuhei Araiso^* Hiroyuki Oshikane^* Shuya Fukai^* Dieter Soll^** Ryuichiro Ishitani^* Osamu Nureki^*

The 21st amino acid selenocysteine is present at the active site of selenoproteins. Selenoprotein synthesis requires translational recoding of the UGA termination codon to a selenocysteine insertion signal (SECIS). At the 3' UTR region on mRNA, there is a hairpin structure called SECIS element which is recognized by EF-Tu homologous EFSec and several related proteins. Thus SeC is incorporated into the translation reaction in ribosome. For SeC-tRNA(SeC) formation, two step reactions are required. First, tRNA(SeC) is acylated with O-phosphoserine (Sep) but not with selenocysteine directly by SepRS to form Sep-tRNA(SeC),which is then converted to SeC-tRNA(SeC) by SepSeCysS. Archaeal SepSeCysS was crystallized using the hanging-drop vapour-diffusion method. Multiwavelength anomalous dispersion (MAD) data set was collelcted using selonomethionyl-substituted crystals. The diffraction data set was collected at 2.5 Å resolution using synchrotron radiation and cryo-cooling. Crystals belong to the primitive monoclinic space group P2₁, with unit-cell parameters a = 75.7, b = 108.1, c = 110.4 Å. There are four SepSeCysS molecules in the asymmetric unit. Now, we are trying to determine the structure of SepSeCysS by the multiwavelength anomalous dispersion method.