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 enzym es 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 isope ntenyl transferase and the product of the CCA1 gene is ATP (CTP): tRNA nucleotidyltransferase. N-2,N-2 dimethylguanosine tRNA methyltransfer ase 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 eff iciency with which these gene products reach mitochondria. Depending o n the gene, ATGs choice is by transcription start site selection, by t ranslational selection or by an interplay between these two processes. A short amino acid sequence is necessary and sufficient for the nucle ar targeting of the dimethylguanosine transferase. There is a good can didate sequence for a nuclear targeting signal (NTS) for the isopenten yl pyrophosphate: tRNA isopentenyl transferase. There are no obvious c andidate sequences for a NTS in the CCA1 sequence.