Transfer RNAs (tRNAs) are synthesized by RNA-polymerase III in the nucleus and modified by multiple enzymes before entering the protein translation process. In recent years intron-containing tRNAs have been detected in all three kingdoms of life. Splicing of intron-containing tRNAs is accomplished by the well-conserved splicing endonuclease (SEN) complex. Although splicing of intron-containing tRNAs is an essential process, it has been discovered that the localization of the SEN complex differs significantly between species.

In eukaryotes, the SEN complex comprises four essential subunits, with Sen2 and Sen34 forming the catalytically active site and, Sen15 and Sen54 forming structural components of the complex. Surprisingly, in humans and vertebrates, it was discovered that the SEN complex is located in the nucleus whereas tRNA splicing in the budding and fission yeast ascomycetes takes place on the cytosolic surface of mitochondria. Despite its high grade of conservation, this finding indicates that species-specific mechanisms evolved to ensure subcellular trafficking of intron-containing tRNAs and the SEN complex.

The basidiomycete fungus Ustilago maydis has emerged as a new model system for cell biology. The genome-wide analysis demonstrated that U. maydis is more closely related to humans than to budding yeast and therefore offers a unique system to study mammalian processes. Interestingly, almost all tRNAs in U. maydis contain introns and sequence identities of SEN complex subunits with a human range between 28% to 50%, indicating a very high degree of conservation between the two organisms. Using state-of-the-art life-cell imaging coupled with genetic engineering we unravel the subcellular localization of the SEN complex in U. maydis, which might help to understand tRNA biology and its associated human diseases in the future.