The indirect incorporation of pyrrolidone carboxylic acid into transfer ribonucleic acid

Abstract

Pyrrolidone carboxylic acid (PCA) is the N-terminal amino acid of several proteins. Since PCA contains a blocked amino group and a structure about its α-carbon similar to that of formylmethionine, an investigation was begun to explore the possibility that PCA might be an initiator amino acid for mammalian protein synthesis. Radioactivity from PCA was shown to be incorporated into tRNA, and some of this radioactivity was in the form of PCA. Experiments using doubly labelled mixtures of PCA and glutamic acid, others testing competition for incorporation between PCA and glutamic acid, and the use of an enzyme preparation in which decyclization is inhibited by mercaptoethanol, demonstrated that PCA cannot be incorporated directly into tRNA, but must first be converted to glutamic acid. An enzyme which serves to decyclize PCA, but which was not a cyclase for glutamic acid, was detected. Since the PCA which was shown to be bound to tRNA had been derived from glutamyl-tRNA, a cyclase mechanism, which did not appear to be artifactual, was present in the pH 5 enzyme mixture. It was concluded that, although PCA cannot be incorporated directly into tRNA, the possibility that PCA serves as an initiator has neither been negated nor proved. The stage in the lifetime of a protein at which it acquires a terminal PCA in place of a terminal glutamine or glutamic acid has not yet been determined.