INTERPLAY OF HETEROGENEOUS TRANSCRIPTIONAL START SITES AND TRANSLATIONAL SELECTION OF AUGS DICTATE THE PRODUCTION OF MITOCHONDRIAL AND CYTOSOLIC NUCLEAR TRANSFER-RNA NUCLEOTIDYLTRANSFERASE FROM THE SAME GENE IN YEAST

ATP (CTP):tRNA nucleotidyltransferase catalyzes the addition of the CC A end to tRNAs. In yeast, nucleotidyltransferase is encoded by the CCA 1 gene and is localized to three cellular compartments: mitochondria, nucleus, and cytosol. There are three in-frame ATGs near the 5' end of the CCA1 open reading frame. Primer extension experiments show multip le transcription initiation sites upstream of ATG1 and between ATG1 an d ATG2. Fractionation of cells carrying a CCA1-COXIV fusion gene demon strates that all three in frame AUGs are used as sites of initiation o f translation. Therefore, both transcription of CCA1 mRNA with heterog eneous 5' ends and translation from downstream AUGs in CCA1 mRNAs play a role in the synthesis of three nucleotidyltransferase isozymes. Pro tein initiating from AUG1 is required for mitochondrial protein synthe sis and, like many other proteins targeted to mitochondria, it is proc essed at the amino terminus upon import into the organelle. The shorte r proteins arising from AUG2 and AUG3 provide nuclear/cytosol activity .