Abstract
Sulfur is an essential element in all living organisms. In tRNA molecules, there are many sulfur-containing nucleosides, introduced post-transcriptionally, that function to ensure proper codon recognition or stabilization of tRNA structure, thereby enabling accurate and efficient translation. The biosynthesis of tRNA sulfur modifications involves unique sulfur trafficking systems that are closely related to cellular sulfur metabolism, and “modification enzymes” that incorporate sulfur atoms into tRNA. Herein, recent biochemical and structural characterization of the biosynthesis of sulfur modifications in tRNA is reviewed, with special emphasis on the reaction mechanisms of modification enzymes. It was recently revealed that TtuA/Ncs6-type 2-thiouridylases from thermophilic bacteria/archaea/eukaryotes are oxygen-sensitive iron-sulfur proteins that utilize a quite different mechanism from other 2-thiouridylase subtypes lacking iron-sulfur clusters such as bacterial MnmA. The various reaction mechanisms of RNA sulfurtransferases are also discussed, including tRNA methylthiotransferase MiaB (a radical S-adenosylmethionine-type iron-sulfur enzyme) and other sulfurtransferases involved in both primary and secondary sulfur-containing metabolites.
Highlights
Transfer RNA is an essential adaptor molecule that bridges genomic information from mRNAs to amino acid sequences in proteins
More than 100 post-transcriptional modifications of Transfer RNA (tRNA) have been identified (Cantara et al, 2011; Väre et al, 2017; Boccaletto et al, 2018), among which sulfur modifications are especially important for tRNA functions
Four kinds of thionucleoside derivatives are found in tRNAs (Figures 1A,B): 4-thiouridine (s4U) at positions 8 and 9 (Lipsett, 1965; Singer and Smith, 1972; Griffey et al, 1986), 2-thiocytidine (s2C) at position (Carbon et al, 1968; Murao et al, 1972), 2-thiouridine (s2U) at position (Crain et al, 2002), 2-thiouridine derivatives at positions (Carbon et al, 1968; Oashi et al, 1970) and 54 (Watanabe et al, 1974), and 2-methylthioadenosine derivatives at position 37 (Burrows et al, 1968; Ishikura et al, 1971)
Summary
In tRNA molecules, there are many sulfur-containing nucleosides, introduced post-transcriptionally, that function to ensure proper codon recognition or stabilization of tRNA structure, thereby enabling accurate and efficient translation. The biosynthesis of tRNA sulfur modifications involves unique sulfur trafficking systems that are closely related to cellular sulfur metabolism, and “modification enzymes” that incorporate sulfur atoms into tRNA. Recent biochemical and structural characterization of the biosynthesis of sulfur modifications in tRNA is reviewed, with special emphasis on the reaction mechanisms of modification enzymes. It was recently revealed that TtuA/Ncs6-type 2-thiouridylases from thermophilic bacteria/archaea/eukaryotes are oxygen-sensitive iron-sulfur proteins that utilize a quite different mechanism from other 2-thiouridylase subtypes lacking iron-sulfur clusters such as bacterial MnmA. The various reaction mechanisms of RNA sulfurtransferases are discussed, including tRNA methylthiotransferase MiaB (a radical S-adenosylmethionine-type iron-sulfur enzyme) and other sulfurtransferases involved in both primary and secondary sulfur-containing metabolites
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