Subcellular localization of the tRNA processing enzyme, tRNA nucleotidyltransferase, in Xenopus laevis oocytes and in somatic cells.

Earlier studies of the tRNA processing enzyme, tRNA nucleotidyltransferase, in rat liver cells suggested that the enzyme was absent from nuclei, whereas more recent studies with tRNA transcription-processing systems from Xenopus laevis oocytes indicated that the enzyme was present in germinal vesicles of these cells. In order to resolve this apparent discrepancy, the subcellular localization of tRNA nucleotidyltransferase in oocytes was directly determined, and its distribution in somatic cells was re-examined. Oocyte germinal vesicles, isolated manually, contained about 25% of the total tRNA nucleotidyltransferase activity of the cell, but the enzyme rapidly leaked out of these nuclei during storage. In contrast, purified nuclei isolated from rat or Xenopus liver using a variety of aqueous procedures were devoid of tRNA nucleotidyltransferase activity. Since rapidly-isolated, crude nuclear preparations from liver contained substantially more of the enzyme than could be accounted for by cytoplasmic contamination, it appears that the enzyme leaks out of the somatic cell nuclei during the lengthy times required for their purification. The implications of these results for the development of tRNA processing systems from somatic cell nuclei are discussed.