Identification and Partial Characterization of the Selenoproteome in Emiliania huxleyi

Many organisms from each of the three domains of life, including the cosmopolitan unicellular marine alga Emiliania huxleyi, require the trace element selenium for growth. Selenium is incorporated into the active site of select proteins known as selenoproteins, in the form of the 21^st amino acid selenocysteine (Sec). Selenoprotein transcripts are characterized by: 1) an in-frame TGA stop codon which codes for the selenocysteine, and 2) a stem loop structure, which refers to the Sec insertion sequence (SECIS) element, present either in the 3’ untranslated region or in the coding region immediately downstream of the TGA codon to direct the ribosomal machinery. Although the function of many selenoproteins remains unknown, many selenoproteins are oxidoreductases involved in redox homeostasis and anti-oxidant defense systems.

We sought to identify and partially characterize the selenoproteome in the marine coccolithophorid, E. huxleyi. A two pronged approach was used: 1) a BLAST homology search was performed to search the genome for known selenoproteins present in SelenoDB (http://www.selenodb.org), and 2) a computer pattern matching algorithm was executed to detect SECIS elements across the genome and identify corresponding selenoprotein genes. The BLAST homology search resulted in the identification of 49 selenoproteins and also revealed the presence of many of the essential components of Sec tRNA biosynthesis pathway and for the translational insertion of Sec, including the selenocysteine synthase (SelA), and SECIS binding protein (SBP2). Manual curation and experimental evidence based on electromobility shift assays and in-vitro transcription-translation reactions have enabled us to validate some of these selenoproteins.