A pyridoxal 5′-phosphate enzyme identified in Escherichia coli. Recognises specifically tRNASec-species. Binding of tRNASec also occurs in the absence of the seryl group. 2-Aminoacryloyl-tRNA, bound to the enzyme as an imine with the pyridoxal phosphate, is an intermediate in the reaction. Since the selenium atom replaces oxygen in serine, the product may also be referred to as L-selenoseryl-tRNASec. The symbol Sel has also been used for selenocysteine but Sec is preferred.