Compared to bacteria, archaea and eukaryotes employ an additional enzyme for the biosynthesis of selenocysteine (Sec), the 21 st natural amino acid (aa). An essential RNA-dependent kinase, O -phosphoseryl-tRNA Sec kinase (PSTK), converts seryl-tRNA Sec to O -phosphoseryl-tRNA Sec , the immediate precursor of selenocysteinyl-tRNA Sec . The sequence of Methanocaldococcus jannaschii PSTK (MjPSTK) suggests an N-terminal kinase domain (177 aa) followed by a presumed tRNA binding region (75 aa). The structures of MjPSTK complexed with ADP and AMPPNP revealed that this enzyme belongs to the P-loop kinase class, and that the kinase domain is closely related to gluconate kinase and adenylate kinase. ATP is bound by the P-loop domain (residues 11–18). Formed by antiparallel dimerization of two PSTK monomers, the enzyme structure shows a deep groove with positive electrostatic potential. Located in this groove is the enzyme's active site, which biochemical and genetic data suggest is composed of Asp-41, Arg-44, Glu-55, Tyr-82, Tyr-83, Met-86, and Met-132. Based on structural comparison with Escherichia coli adenylate kinase a docking model was generated that assigns these amino acids to the recognition of the terminal A76-Ser moieties of Ser-tRNA Sec . The geometry and electrostatic environment of the groove in MjPSTK are perfectly complementary to the unusually long acceptor helix of tRNA Sec .