A Specific Hepatic Transfer RNA for Phosphoserine

Radioactive O-phosphoryl-L-serine was detected after alkaline deacylation of rat and rooster liver [(3)H]seryl-tRNA acylated in vitro with homologous synthetases. Ribonuclease treatment of this tRNA yielded a compound with the properties of phosphoseryl-adenosine. Benzoylated DEAE-cellulose chromatography of seryl-tRNA yielded four distinct peaks, only one of which contained phosphoserine. A unique fraction for phosphoserine was also found on chromatography of nonacylated tRNA. In ribosome binding studies, this fraction responded very slightly with poly(U,C), but not with any of the known serine trinucleotide codons. Substantial incorporation of [(3)H]-serine into protein from this tRNA species was observed in an aminoacyl-tRNA dependent polysomal system derived from chick oviducts. No phosphoserine was found in Escherichia coli or yeast seryl-tRNA acylated with homologous enzymes, nor in E. coli seryl-tRNA acylated with liver synthetase. In the absence of tRNA, free phosphoserine was not formed in reaction mixtures, which suggests that phosphoseryl-tRNA arises by phosphorylation of the unique seryl-tRNA species. These results demonstrate a discrete tRNA(Ser) species in rat and rooster liver containing phosphoserine and suggest that this tRNA is involved in ribosomal polypeptide synthesis.