Export of Precursor tRNAIle from the Nucleus to the Cytoplasm in Human Cells

In the current concept, tRNA maturation in vertebrate cells, including splicing of introns, trimming of 5’ leader and 3’ trailer, and adding of CCA, is thought to occur exclusively in the nucleus. Here we provide evidence to challenge this concept. Unspliced intron-containing precursor tRNA Ile was identified in Human Immunodeficiency Virus type 1 (HIV-1) virions, which are synthesized in the cytoplasm. Northern blot, confocal microscopy and quantitative RT-PCR further verified enrichment of this unspliced tRNA Ile within the cytoplasm in human cells. In addition to containing an intron, the cytoplasmic precursor tRNA Ile also contains a short incompletely processed 5´ leader and a 3´ trailer, which abundance is around 1000 fold higher than the nuclear precursor tRNA Ile with long 5’ leader and long 3’ trailer. In vitro data also suggest that the cytoplasmic unspliced end-immature precursor tRNA Ile could be processed by short isoform of RNase Z, but not long isoform of RNase Z. These data suggest that precursor tRNAs could export from the nucleus to the cytoplasm in human cells, instead of be processed only in the nucleus.