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.