Distinct genetic code expansion strategies for selenocysteine and pyrrolysine are reflected in different aminoacyl-tRNA formation systems

Selenocysteine, pyrrolysine, and the unique energy metabolism of methanogenic archaea

Methanogenic archaea are a group of strictly anaerobic microorganisms characterized by their strict dependence on the process of methanogenesis for energy conservation. Among the archaea, they are also the only known group synthesizing proteins containing selenocysteine or pyrrolysine. All but one of the known archaeal pyrrolysine-containing and all but two of the confirmed archaeal selenocysteine-containing protein are involved in methanogenesis. Synthesis of these proteins proceeds through suppression of translational stop codons but otherwise the two systems are fundamentally different. This paper highlights these differences and summarizes the recent developments in selenocysteine- and pyrrolysine-related research on archaea and aims to put this knowledge into the context of their unique energy metabolism.

A tRNA-dependent cysteine biosynthesis enzyme recognizes the selenocysteine-specific tRNA in Escherichia coli

The essential methanogen enzyme Sep-tRNA:Cys-tRNA synthase (SepCysS) converts O-phosphoseryl-tRNA(Cys) (Sep-tRNA(Cys)) into Cys-tRNA(Cys) in the presence of a sulfur donor. Likewise, Sep-tRNA:Sec-tRNA synthase converts O-phosphoseryl-tRNA(Sec) (Sep-tRNA(Sec)) to selenocysteinyl-tRNA(Sec) (Sec-tRNA(Sec)) using a selenium donor. While the Sep moiety of the aminoacyl-tRNA substrates is the same in both reactions, tRNA(Cys) and tRNA(Sec) differ greatly in sequence and structure. In an Escherichia coli genetic approach that tests for formate dehydrogenase activity in the absence of selenium donor we show that Sep-tRNA(Sec) is a substrate for SepCysS. Since Sec and Cys are the only active site amino acids known to sustain FDH activity, we conclude that SepCysS converts Sep-tRNA(Sec) to Cys-tRNA(Sec), and that Sep is crucial for SepCysS recognition.