A selenoprotein in the plant kingdom. Mass spectrometry confirms that an opal codon (UGA) encodes selenocysteine in Chlamydomonas reinhardtii gluththione peroxidase.
Selenoproteins that contain the rare amino acid selenocysteine in their primary structure have been identified in diverse organisms such as viruses, bacteria, archea, and mammals, but so far not in yeast or plants. Among the most thoroughly investigated families of selenoenzymes are the animal glutathione peroxidases (GPXs). In the last few years, genes encoding GPX-like homologues from Chlamydomonas and higher plants have been isolated, but, unlike the animal ones, all of them have cysteine (rather than selenocysteine) residues in their catalytic site. In all organisms investigated that contain selenoproteins, selenocysteine is encoded by a UGA opal codon, which is usually a stop codon. We report here that, in Chlamydomonas reinhardtii, the cDNA-cloned sequence of a GPX homologue contains an internal TGA codon in frame to the ATG. Specific mRNA expression, protein production, and enzyme activity are selenium-dependent. Sequence analysis of the peptides produced by proteolytic digestion, performed by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS), confirmed the presence of a selenocysteine residue at the predicted site and suggest its location in the mitochondria. Thus, our data present the first direct proof that a UGA opal codon is decoded in the plant kingdom to incorporate selenocysteine.
Fu
Lian-Hai
LH
Department of Fruit-Tree Breeding and Molecular Genetics, Agricultural Research Organization, Volcani Center, Bet Dagan 50250, Israel.
Wang
Xiao-Feng
XF
Eyal
Yoram
Y
She
Yi-Min
YM
Donald
Lynda J
LJ
Standing
Kenneth G
KG
Ben-Hayyim
Gozal
G
D000595:Amino Acid Sequence; D000818:Animals; D016825:Chlamydomonas reinhardtii / Q000201:enzymology*; D018388:Codon, Terminator; D018076:DNA, Complementary / Q000032:analysis; D016054:DNA, Protozoan / Q000737:chemistry; D004926:Escherichia coli; D005979:Glutathione Peroxidase / Q000235:genetics*; D013058:Mass Spectrometry; D008969:Molecular Sequence Data; D010449:Peptide Mapping; D012333:RNA, Messenger / Q000378:metabolism; D016053:RNA, Protozoan / Q000737:chemistry; D012643:Selenium / Q000378:metabolism; D017279:Selenocysteine / Q000737:chemistry / Q000235:genetics*; D019032:Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization
D018388:Codon, Terminator; D018076:DNA, Complementary; D016054:DNA, Protozoan; D012333:RNA, Messenger; D016053:RNA, Protozoan; D017279:Selenocysteine; D005979:Glutathione Peroxidase; D012643:Selenium