Abstract

ATP (CTP):tRNA nucleotidyltransferase catalyzes the addition of CMP and AMP residues to the 3 ' ends of tRNAs that have an incomplete CCA sequence. In eukaryotic cells, nuclear, mitochondrial and chloroplast genomes all encode tRNAs. Therefore, although tRNA nucleotidyltransferase is synthesized in the cytosol, it must function in multiple organelles and be targeted to these locations from its site of synthesis. In general, while much is known about the targeting information contained in proteins, less is known about the accessory factors involved in targeting. To identify proteins which could interact with lupin or Arabidopsis tRNA nucleotidyltransferase, the yeast two-hybrid system was employed to screen an Arabidopsis cDNA library. As a result, 11 cDNA clones have been isolated belonging to four different classes of cDNAs. Representatives from each class were sequenced in their entirety. Sequence analysis of these cDNAs revealed a major open reading frame for each cDNA. The predicted amino acid sequence of each cDNA was used to search the GenBank and Arabidopsis databases. One group of clones could encode a 615-amino acid polypeptide showing 54% identity with the Arabidopsis kinesin light chain. Another group could encode a 129-amino acid polypeptide which shows 39% identity with the human KE2 protein. A third group could encode a 180-amino acid polypeptide which shows 58% identity with cucumber raffinose synthase and finally one clone could encode a 188-amino acid polypeptide which shows 37% identity with a hypothetical protein from Synechocystis sp. Mapping of the regions of tRNA nucleotidyltransferase required for interaction demonstrated that the amino-terminal extensions of the tRNA nucleotidyltransferases are required for interaction with all four of these clones.