All cells in an organism contain a copy of DNA in their nucleus. In order to implement the instructions it contains, this DNA must be copied into an RNA molecule, which reaches the ribosomes, which in turn read this information and synthesize the proteins. Codons, triplets of amino acids that make up proteins and are the markers that ribosomes need to know how to make each protein, are essential in this transition process. There are a total of 61 codons which encode 20 amino acids and three codons which act as stop signals in the translation process.
However, some organisms use an extra amino acid, selenocysteine, nicknamed the 21st amino acid, which lacks its own codon and uses a stop codon after changing it. For this, it has a complex machinery, with specific enzymes and RNAs; this process can prove to be very costly for the cell. But why? What function does this amino acid have in proteins? Why is it present in humans and in other vertebrates when, on the other hand, other species have lost it? Today, researchers from the Center for Genomic Regulation (CRG) in Barcelona, which is part of the Barcelona Institute of Science and Technology, in collaboration with CRG alumni Marco Mariotti and Vadim N. Gladyshev, from Harvard Medical School (United States), and Gustavo Salinas of the University of the Republic of Uruguay, shed light on these issues.
“In previous studies, we found that the selenocysteine machinery had been lost several times during evolution and we began to be interested in why it disappears so easily in some groups but not in others. others, “says ICREA research professor Toni. Gabaldón, head of the Comparative Genomics group at CRG.
Fungi were the only kingdom of organisms in which a species containing selenocysteine had never been found, and the researchers decided to focus on them, taking advantage of the recent publication of a thousand fungal genomes in databases accessible to the public. By analyzing them, they discovered, as they reported in the article published in Natural microbiology, that nine of the 1,000 species actually possessed this amino acid.
“This surprised us, because no mushroom was believed to contain selenocysteine,” Gabaldón explains, which is why the nine species they discovered that did have it belonged to relatively unsequenced groups of fungi that “diverged at one point. early stage in fungal evolution, which means we will likely find more cases of selenocysteine when more genomes from these groups are sequenced. “
The ancestor of the fungi they identified with this amino acid also had it. Some lines have retained it, while others have lost it, which could also be the case in other organisms. “The question that remains to be resolved is why it is lost in some organisms while in others these genes are essential,” Gabaldón explains. “Understanding why selenocysteine is important in fungi and other branches of the tree of life can help us understand why it is so important to our species and define what makes selenium essential for human health,” concludes- he.
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