Seryl-tRNA synthetase specificity for tRNA^Sec in Bacterial Sec biosynthesis

tRNA synthetases are responsible for decoding the molecular information, from codons to amino acids. Seryl-tRNA synthetase (SerRS), besides the five isoacceptors of tRNA^Ser, recognizes tRNA^[Ser]Sec for the incorporation of selenocysteine (Sec, U) into selenoproteins. The selenocysteine synthesis pathway is known and is dependent on several protein-protein and protein-RNA interactions. Those interactions are not fully described, in particular, involving tRNA^[Ser]Sec and SerRS. Here we describe the molecular interactions between the Escherichia coli Seryl-tRNA synthetase (EcSerRS) and tRNA^[Ser]Sec in order to determine their specificity, selectivity and binding order, leading to tRNA aminoacylation. The dissociation constant of EcSerRS and tRNA^[Ser]Sec was determined as (126 ± 20) nM. We also demonstrate that EcSerRS binds initially to tRNA^[Ser]Sec in the presence of ATP for further recognition by E. coli selenocysteine synthetase (EcSelA) for Ser to Sec conversion. The proposed studies clarify the mechanism of tRNA^[Ser]Sec incorporation in Bacteria as well as of other domains of life.