pGp as the main product of bovine tRNA kinase

One of the Ser-tRNAs, Ser-tRNA(Sec), is converted to Sec-tRNA(Sec) by Sec synthase. This Ser-tRNA(Sec) is also converted to phosphoser-tRNA(Sec) by tRNA kinase. In this study, we analyzed of the products of phosphorylation with tRNA kinase. [3H]Ser-tRNA(Sec) purified on Sephacryl S-200 was phosphorylated with [gamma-32P]ATP by tRNA kinase. The product [32P][3H]phosphoser-tRNA was purified on Sephacryl S-200 and hydrolyzed with ribonuclease T2. The chromatogram of this hydrolyzate on DEAE-cellulose in 7 M urea buffer showed four peaks. The first peak of the pass-through fraction was seryl-adenosine liberated from the 3'-terminal of the tRNA. The second peak, eluted before the third peak containing inorganic phosphate, was phosphoseryl-adenosine. The major compound in the fourth peak was pGp. As a control experiment, non-acylated tRNA(Sec) was used as a substrate of phosphorylation and the product was analyzed. The chromatogram of the digest with ribonuclease T2 showed no peak of phosphoseryl-adenosine, but a peak of pGp was seen with the peak of inorganic phosphate. Thus, the major product in the presence of tRNA kinase was pGp, and a small but significant proportion of the radioactivity was found as phosphoserine in the presence of seryl residue on the 3'-CCA terminal of tRNA(Sec). These results indicated that tRNA kinase phosphorylates not only Ser-tRNA to phosphoser-tRNA but also Gp of the 5'-termini of tRNA to pGp. This study gives a new role to mammalian tRNA kinase.