Closely related CC- and A-adding enzymes collaborate to construct and repair the 3'-terminal CCA of tRNA in Synechocystis sp. and Deinococcus radiodurans.

The 3'-terminal CCA sequence of tRNA is faithfully constructed and repaired by the CCA-adding enzyme (ATP(CTP):tRNA nucleotidyltransferase) using CTP and ATP as substrates but no nucleic acid template. Until recently, all CCA-adding enzymes from all three kingdoms appeared to be composed of a single kind of polypeptide with dual specificity for adding both CTP and ATP; however, we recently found that in Aquifex aeolicus, which lies near the deepest root of the eubacterial 16 S rRNA-based phylogenetic tree, CCA addition represents a collaboration between closely related CC-adding and A-adding enzymes (Tomita, K. and Weiner, A. M. (2001) Science 294, 1334-1336). Here we show that in Synechocystis sp. and Deinococcus radiodurans, as in A. aeolicus, CCA is added by homologous CC- and A-adding enzymes. We also find that the eubacterial CCA-, CC-, and A-adding enzymes, as well as the related eubacterial poly(A) polymerases, each fall into phylogenetically distinct groups derived from a common ancestor. Intriguingly, the Thermatoga maritima CCA-adding enzyme groups with the A-adding enzymes, suggesting that these distinct tRNA nucleotidyltransferase activities can intraconvert over evolutionary time.