TRANSCRIPTION OF CLONED TRANSFER-RNA GENES AND THE NUCLEAR PARTITIONING OF A TRANSFER-RNA PRECURSOR

The transcription of tRNA genes (tDNA) and processing of the transcripts were studied by injecting cloned tDNA into Xenopus oocyte nuclei. Three main conclusions can be drawn. First, eukaryotic nuclear tRNA genes, but neither prokaryotic nor mitochondrial tRNA genes, are expressed in injected oocytes. While both nematode and yeast tDNA direct the synthesis of authentic tRNA, neither Escherichia coli tDNA nor human mitochondrial tDNA support the synthesis of defined tRNA when injected into oocytes. Second, competition experiments with coinjected 5S genes and inhibition experiments with .alpha.-amanitine show that injected tDNA are transcribed by RNA polymerase III. Third, oocytes injected with a nematode tDNA synthesize a tRNA precursor which is processed post-transcriptionally by removal of a 5'' leader sequence. This precursor is found exclusively in the nucleus and is processed in the nucleus before the mature tRNA enters the cytoplasm.