Journal
SCIENCE
Volume 340, Issue 6128, Pages 75-78Publisher
AMER ASSOC ADVANCEMENT SCIENCE
DOI: 10.1126/science.1229521
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Funding
- Japan Society for the Promotion of Science (JSPS)
- Alexander von Humboldt Foundation (Bonn, Germany)
- JSPS
- Ministry of Education, Culture, Sports, Science and Technology
- Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy [DE-FG02-98ER20311]
- National Institute of General Medical Sciences [GM22854]
- Defense Advanced Research Projects Agency [N66001-12-C-4020, N66001-12-C-4211]
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The 21st amino acid, selenocysteine (Sec), is synthesized on its cognate transfer RNA (tRNA(Sec)). In bacteria, SelA synthesizes Sec from Ser-tRNA(Sec), whereas in archaea and eukaryotes SepSecS forms Sec from phosphoserine (Sep) acylated to tRNA(Sec). We determined the crystal structures of Aquifex aeolicus SelA complexes, which revealed a ring-shaped homodecamer that binds 10 tRNA(Sec) molecules, each interacting with four SelA subunits. The SelA N-terminal domain binds the tRNA(Sec)-specific D-arm structure, thereby discriminating Ser-tRNA(Sec) from Ser-tRNA(Ser). A large cleft is created between two subunits and accommodates the 3'-terminal region of Ser-tRNA(Sec). The SelA structures together with in vivo and in vitro enzyme assays show decamerization to be essential for SelA function. SelA catalyzes pyridoxal 5'-phosphate-dependent Sec formation involving Arg residues nonhomologous to those in SepSecS. Different protein architecture and substrate coordination of the bacterial enzyme provide structural evidence for independent evolution of the two Sec synthesis systems present in nature.
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