The ATP(CTP):tRNA nucleotidyltransferase (CCA-adding enzyme) adds CCA to the 3' end of immature or damaged tRNAs. It is reported on here the cloning, expression analysis, and functional characterization of the Xenopus CCA-adding enzyme; XCCA (GenBank Accession #AF466151). It is demonstrated that XCCA adds cytosine and adenosine residues to the ends of prepared tRNA and is therefore a functional CCA-adding enzyme. XCCA is encoded by a rare mRNA present at less than 0.001% of the cellular mRNA in all adult tissues examined. The mRNA is expressed as two transcripts of 1.5 and 2.3 kb, generated through differential utilization of two transcription start sites and two 3' cleavage and polyadenylation sites. Utilization of the most 5' transcription initiation site produces an mRNA encoding a putative mitochondrial import sequence. It is anticipated that the Xenopus oocyte will be an excellent system for analyzing the regulation of XCCA expression and the intracellular targeting of the XCCA enzyme. (C) 2002 Elsevier Science (USA). All rights reserved.