Kinetoplastids encode a single nuclear tryptophanyl tRNA that contains a CCA anticodon able to decode the UGG codons used in cytoplasmic protein synthesis but cannot decode the mitochondrial UGA codons. Following mitochondrial import, this problem is circumvented in Trypanosoma brucei by specifically editing the tRNA Trp anticodon to UCA, which can now decode the predominant mitochondrial UGA tryptophan codons. This tRNA also undergoes an unusual thiolation at position 33 of the anticodon loop, the only known modification at U33 in any tRNA. In other organisms, tRNA thiolation is mediated by the cysteine desulfurase, Nfs1 (IscS). However, T. brucei encodes two Nfs homologues, one cytoplasmic and the other mitochondrial. We show by a combination of RNA interference and Northern and Western analyses that the mitochondria-targeted TbNfs, and not TbNfs-like protein, is essential for thiolation of both cytosolic and mitochondrial tRNAs. Given the exclusive mitochondrial localization of TbNfs, how it mediates thiolation in the cytoplasm remains unclear. Furthermore, thiolation specifically affects thiolated tRNA stability in the cytoplasm but more surprisingly acts as a negative determinant for the essential C to U editing in T. brucei. This provides a first line of evidence for mitochondrial C to U editing regulation in this system.

Nfs‐like proteins have cysteine desulfurase (CysD) activity, which removes sulfur (S) from cysteine, and provides S for iron–sulfur cluster assembly and the thiolation of tRNAs. These proteins also have selenocysteine lyase activity in vitro, and cleave selenocysteine into alanine and elemental selenium (Se). It was shown previously that the Nfs‐like protein called Nfs from the parasitic protist Trypanosoma brucei is a genuine CysD. A second Nfs‐like protein is encoded in the nuclear genome of T. brucei. We called this protein selenocysteine lyase (SCL) because phylogenetic analysis reveals that it is monophyletic with known eukaryotic selenocysteine lyases. The Nfs protein is located in the mitochondrion, whereas the SCL protein seems to be present in the nucleus and cytoplasm. Unexpectedly, downregulation of either Nfs or SCL protein leads to a dramatic decrease in both CysD and selenocysteine lyase activities concurrently in the mitochondrion and the cytosolic fractions. Because loss of Nfs causes a growth phenotype but loss of SCL does not, we propose that Nfs can fully complement SCL, whereas SCL can only partially replace Nfs under our growth conditions. Structured digital abstract http://mint.bio.uniroma2.it/mint/search/interaction.do?interactionAc=MINT-7298305: NFS (uniprotkb:http://www.uniprot.org/uniprot/Q386Y7?format=text&ascii) and PHB1 (uniprotkb:http://www.uniprot.org/uniprot/Q57UX1?format=text&ascii) colocalize (http://www.ebi.ac.uk/ontology-lookup/?termId=MI:0403) by cosedimentation through density gradients (http://www.ebi.ac.uk/ontology-lookup/?termId=MI:0029) http://mint.bio.uniroma2.it/mint/search/interaction.do?interactionAc=MINT-7298357: SCL (uniprotkb:http://www.uniprot.org/uniprot/Q38DC4?format=text&ascii) and Enolase (uniprotkb:http://www.uniprot.org/uniprot/Q38BV6?format=text&ascii) colocalize (http://www.ebi.ac.uk/ontology-lookup/?termId=MI:0403) by cosedimentation through density gradients (http://www.ebi.ac.uk/ontology-lookup/?termId=MI:0029)