HOW SINGLE GENES PROVIDE TRANSFER-RNA PROCESSING ENZYMES TO MITOCHONDRIA, NUCLEI AND THE CYTOSOL

TRM1, MOD5 and CCA1 are yeast genes that provide tRNA processing enzymes to mitochondria and the nuclear/cytosolic compartments. The product of the TRM1 gene is N-2,N-2 dimethylguanosine tRNA methyltransferase. The product of the MOD5 gene is isopentenyl pyrophosphate: tRNA isopentenyl transferase and the product of the CCA1 gene is ATP (CTP): tRNA nucleotidyltransferase. N-2,N-2 dimethylguanosine tRNA methyltransferase is found in the mitochondria and the nucleus. The tRNA isopentenyl transferase and tRNA nucleotidyltransferase are found in mitochondria, nuclei and the cytosol. Genes coding for these three enzymes contain more than one in-frame ATG. Where translation begins dictates the efficiency with which these gene products reach mitochondria. Depending on the gene, ATGs choice is by transcription start site selection, by translational selection or by an interplay between these two processes. A short amino acid sequence is necessary and sufficient for the nuclear targeting of the dimethylguanosine transferase. There is a good candidate sequence for a nuclear targeting signal (NTS) for the isopentenyl pyrophosphate: tRNA isopentenyl transferase. There are no obvious candidate sequences for a NTS in the CCA1 sequence.