Investigation of organisms having unique selenium metabolic pathways and its application to bioremediation

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Selenium is an essential trace element with a variety of physiological role and is strictly discriminated from its cognate sulfur to be incorporated into a specific position of proteins as a selenocysteine residue. In this study, we investigated organisms having unique selenium metabolism pathway and analyzed the structure and function of novel enzymes involved in selenium metabolism.1. It is suggested that the synthesis of O-phosphoseryl-tRNA from seryl-tRNA by the catalysis of O-phosphosery1-tRNA kinase, (PSTK) is essential for the biosynthesis of archaeal selenocystey1-tRNA. However, our in vitro study with PAD, PSTK, and selenocysteine synthase (SecS) from the methanogenic thermophile Methanocaldococcus jannaschii revealed that the synthesis of selenocystey1-tRNA is able to proceed via two independent pathways: one is dependent on PSTK and SelD, and another depends on SecS and SelD.2. We searched far a bacterium with a unique selenoprotein and found that Geobacter sulfurreducens probably contains a novel c-type cytochrome-like selenoprotein. This protein termed GSU2937-GSU2936 has 5 CXXCH motifs in its N-terminal region, which is encoded by the ORF upstream of a UGA codon for selenocysteine insertion. The GSU2937-GSU2936 protein shows only less than 12% sequence identity with cytochrome c nitrite reductase and hydroxylamine oxidoreductase, which are known as multi-heme proteins, suggesting that it belongs to a novel member of a protein family.