Covalent modification of phenylalanyl-tRNA synthetase with phenylalanine during the amino acid activation reaction catalyzed by the enzyme.

Yeast phenylalanyl-tRNA synthetase (PRS) is shown to undergo autoaminoacylation with phenylalanine under in vitro amino acid activation conditions. Phenylalanyl adenylate enzyme complex yields a covalent phenylalanyl isopeptide exclusively with the beta subunit of the alpha 2 beta 2 enzyme. Contrary to previously reported cases of autoaminoacylation of aspartyl-tRNA synthetase and tryptophanyl-tRNA synthetase, the autoaminoacylation of PRS occurs under a specific set of conditions and results in the identification of only one labeled tryptic peptide on two types of high pressure liquid chromatography columns. The ability of PRS to undergo this covalent modification directly correlates with its ability to catalyze the synthesis of diadenosine 5',5"'-P1,P4-tetraphosphate from enzyme-bound phenylalanyl adenylate. Both reactions require the presence of low levels of zinc or cadmium and are inhibited by tRNAPhe or by low levels of low molecular weight thiols. Since diadenosine 5',5"'-P1,P4-tetraphosphate synthesis is known to be catalyzed in vivo in response to oxidation stress, it is also likely that the autoaminoacylation of phenylalanyl-tRNA synthetase may occur in vivo under a similar set of conditions. These reactions are thus not simply the result of accumulation of phenylalanyl adenylate and probably reflect conformational changes in the protein which are brought about by its interaction with zinc or cadmium.