Leishmania donovani encodes a functional selenocysteinyl-tRNA synthase [Gene Regulation]

November 19th, 2015 by Manhas, R., Gowri, V. S., Madhubala, R.

The synthesis of selenocysteine, the 21st amino acid occurs on its transfer RNA (tRNA), tRNASec. tRNASec is initially aminoacylated with serine by seryl-tRNA synthetase and the resulting seryl moiety is converted to phosphoserine by O-phosphoseryl-tRNA kinase (PSTK) in eukaryotes. The selenium donor, selenophosphate is synthesized from selenide and ATP by selenophosphate synthetase. Selenocysteinyl-tRNA synthase (SepSecS) then uses the O-phosphoseryl-tRNASec and selenophosphate, to form Sec-tRNASec in eukaryotes. Here, we report the characterization of selenocysteinyl-tRNA synthase from L. donovani. Kinetoplastid SepSecS enzymes are phylogenetically closer to worm SepSecS. LdSepSecS was found to exist as a tetramer. Leishmania SepSecS enzyme was found to be active and able to complement the ∆selA deletion in E. coli JS1 strain only in the presence of archaeal PSTK, indicating the conserved nature of the PSTK-SepSecS pathway. LdSepSecS was found to localize in the cytoplasm of the parasite. Gene deletion studies indicate that Leishmania SepSecS is dispensable for the parasite survival. The parasite was found to encode three selenoproteins; which expressed only in the presence of SepSecS. Selenoproteins of L. donovani are not required for the growth of the promastigotes. Auranofin, a known inhibitor of selenoprotein synthesis showed same sensitivity towards the wild-type and the null mutants suggesting its effect is not through binding to selenoproteins. The 3-D structural comparison indicates that the human and Leishmania homologs are structurally highly similar but their association modes leading to tetramerization seem different