What is selenocysteine? Are there really only 20 proteinogenic amino acids? How can more than 20 amino acids fit into the genetic code? Find the answers in this review of: Gene for a novel tRNA species that accepts L-serine and cotranslationally inserts selenocysteine. Leinfelder W, Zehelein E, Mandrand-Berthelot MA, Böck A. Nature. 1988 Feb 25;331(6158):723-5. doi: 10.1038/331723a0.

Twitter: [ Ссылка ]

OTHER VIDEOS YOU MIGHT LIKE:
• Ribosomes know why the genetic code is non-overlapping (Brenner, 1957) - [ Ссылка ]
• Genomic imprinting: It takes two to make it outta sight (Barton et al., 1991) - [ Ссылка ]
• Understanding the discovery of RNAi, a key regulator of gene expression (Fire et al., 1998) - [ Ссылка ]

You only need 21 amino acids to create the basis for every protein that keeps you alive. 20 of those amino acids are “standard”… what’s so special about the 21st? Let’s explore the strange story of selenocysteine – the way the cell has managed to squeeze in a bonus amino acid into the genetic code.

tRNA is the ‘translator’ between two languages of genetic information: the bases of RNA, and the amino acids of protein. With the help of tRNA, a triplet of bases (a codon) is converted into 1 amino acid. By 1967, we knew exactly what every codon coded for, and that gave us 20 unique amino acids. Everything was perfect, right?

Well, in 1976, we found selenocysteine in a protein, and after that we found more. Was there a 21st amino acid that was being used in these selenoproteins? The answer was yes – the UGA codon somehow means ‘stop translation’ AND selenocysteine! In 1988, we discovered that there is, in fact, a tRNA for selenocysteine, but structurally it was very different to the other tRNAs. On top of this, the tRNA was being charged with serine, making even less sense.

The discovery of the SECIS (selenocysteine insertion sequence) helped clear this fog. Essentially, whether a SECIS is present determines what the UGA indicates. Further, the tRNA is likely charged with serine because free-floating selenocysteine would disrupt cellular machinery. As for the evolution of selenocysteine, we are not so sure. Why did such an elaborate work-around to normal translation for one extra option appear? Was UGA even supposed to be a stop codon in the first place? There is still debate. Either way, we need selenoproteins for a healthy life. For now, selenocysteine remains a peculiar consequence of evolution.

Creator: Joshua Khoo

References:
Böck, A., C. Baron, K. Forchhammer, J. Heider, W. Leinfelder et al., 1990 From nonsense to sense: UGA encodes selenocysteine in formate dehydrogenase and other selenoproteins. Mol. Basis Bact. Metabol. 41: 61–68.
Brenner, S., L. Barnett, E. R. Katz, and F. H. C. Crick, 1967 UGA: A third nonsense triplet in the genetic code. Nature. 213: 449–450.
Cone, J. E., R. Martín del Río, J. N. Davis, and T. C. Stadtman, 1976 Chemical characterization of the selenoprotein component of clostridial glycine reductase: Identification of selenocysteine as the organoselenium moiety. Proc. Natl. Acad. Sci. 73: 2659–2663.
Flissi, A., E. Ricart, C. Campart, M. Chevalier, Y. Dufresne et al., 2020 Norine: update of the nonribosomal peptide resource. Nucl. Acids Res. 48: D465–D469.
Gonzalez-Flores, J., S. P. Shetty, A. Dubey, and P. R. Copeland, 2013 The molecular biology of selenocysteine. Biomol Concepts. 4: 349–365.
Gromer, S., J. K. Eubel, B. L. Lee, and J. Jacob, 2005 Human selenoproteins at a glance. Cell. Mol. Life Sci. 62: 2414–2437.
Leinfelder, W., E. Zehelein, M. Mandrand-Berthelot, and A. Böck, 1988 Gene for a novel tRNA species that accepts L-serine and cotranslationally inserts selenocysteine. Nature. 331: 723–725.
Ponomarenko, E. A., E. V. Poverennaya, E. V. Ilgisonis, M. A. Pyatnitskiy, A. T. Kopylov et al., 2016 The size of the human proteome: The width and depth. Int. J. Anal. Chem. 2016: 7436849.
Schmidt, R. L., and M. Simonović, 2012 Synthesis and decoding of selenocysteine and human health. Croat. Med. J. 53: 535–550.
Serrão, V. H. B., I. R. Silva, M. T. A. da Silva, J. F. Scortecci, A. F. Fernandes et al., 2018 The unique tRNASec and its role in selenocysteine biosynthesis. Amino Acids. 50: 1145–1167.
Silva, I. R., V. H. B. Serrão, L. R. Manzine, L. M. Faim, M. T. A. da Silva et al., 2015 Formation of a ternary complex for selenocysteine biosynthesis in bacteria. J. Biol. Chem. 290: 29178–29188.
Zinoni, F., A. Birkmann, T. C. Stadtman, and A. Böck, 1986 Nucleotide sequence and expression of the selenocysteine-containing polypeptide of formate dehydrogenase (formate-hydrogen-lyase-linked) from Escherichia coli. Proc. Natl. Acad. Sci. 83: 4650–4654.
Zinoni, F., J. Heider, and A. Böck, 1990 Features of the formate dehydrogenase mRNA necessary for decoding of the UGA codon as selenocysteine. Proc. Natl. Acad. Sci. 87: 4660–4664.

свернуть