O-phosphoseryl-tRNASec kinase (, PSTK, phosphoseryl-tRNA[Ser]Sec kinase, phosphoseryl-tRNASec kinase) is an enzyme with systematic name ATP:L-seryl-tRNASec O-phosphotransferase. This enzyme catalyses the following chemical reaction
In archaea and eukarya selenocysteine formation is achieved by a two-step process.