MECHANISMS LEADING TO AND THE CONSEQUENCES OF ALTERING THE NORMAL-DISTRIBUTION OF ATP(CTP)TRNA NUCLEOTIDYLTRANSFERASE IN YEAST

CCA1 codes for mitochondrial, cytosolic, and nuclear ATP(CTP):tRNA nuc leotidyltransferase. Studies reported here examine the mechanisms lead ing to and the consequences of altering the distribution of this impor tant tRNA processing enzyme. We show that the majority of Cca1p-I, tra nslated from the first in-frame ATG, is in mitochondria but surprising ly, there is a small contribution to nuclear and cytosolic tRNA proces sing by this form as well. The majority of Cca1p-II and Cca1p-III, tra nslated from ATG2 and ATG3, respectively, is in the cytosol but both a re also located in the nucleus for processing precursors. Altering the cytosolic/nuclear distribution of Cca1p by fusing the SV40 nuclear lo calization signal to the 5' end of CCA1 causes a growth defect and res ults in the accumulation of end-shortened tRNAs in the cytosol. These results suggest an important role for Cca1p in the cytosol of eukaryot es, presumably in the repair of 3' CCA termini. These experiments also demonstrate that individual tRNAs are affected differently by reduced cytosolic nucleotidyltransferase and that cells resuming exponential growth are more severely affected than those continuing exponential gr owth.