Evolution of the biosynthesis of 3′-terminal C-C-A residues in T-even bacteriophage transfer RNAs

Abstract

We have characterized the 3′ termini of T-even bacteriophage transfer RNAs synthesized after infection of a cca mutant strain of Escherichia coli deficient in the enzyme ATP(CTP): tRNA nucleotidyltransferase (E.C. 2.7.7.25). Four of eight T4 tRNAs, two of eight T2 tRNAs and one of six T6 tRNAs lacked the 3′ C-C-A sequence after infection of the cca mutant strain. A purified preparation of tRNA nucleotidyltransferase was capable of synthesis of 3′ C-C-A onto the immature tRNAs in vitro. We concluded that under normal physiological conditions tRNA nucleotidyltransferase is required for the biosynthesis of these tRNA molecules. For the remaining tRNAs, the 3′ C-C-A residues apparently arise directly from the tRNA genes during transcription. Comparison of tRNAs induced by bacteriophages T2, T4 and T6 indicated that the biosynthetic origin of the 3′ C-C-A sequence of a given tRNA was usually the same in different bacteriophages. However, two examples were found where the 3′ C-C-A sequence of a given tRNA was formed by transcription in one bacteriophage and by tRNA nucleotidyltransferase in another bacteriophage. Based on these observations, we speculate that primitive tRNAs did not require 3′ C-C-A residues for their protein-synthetic functions, and propose a scheme whereby tRNA genes encoding these residues evolved.