APR 10, 2019 7:22 AM PDT

Selenocysteine, the 21st Amino Acid

WRITTEN BY: Carmen Leitch

Proteins help compose many of the essential parts of an organism, and to make them, a cell must first generate mRNA from the genes in the genome that code for protein. Cellular machines called ribosomes translate the mRNA into proteins, reading three nucleotide bases in the mRNA at a time. Those triplets of bases are called codons; 61 codons translate to twenty amino acids (there is some redundancy), and three codons stop translation. The ribosome reads the sequence of mRNA to determine the appropriate arrangement of amino acids. Strung together, those amino acids eventually create a protein. Learn more about the process from the video. 

There is an unusual 21st amino acid too, called selenocysteine. It is needed for the function of some enzymes. However, it is not coded for directly in the genome. It is formed by the modification of another amino acid - a serine that links to a tRNA molecule (which connects amino acids to mRNA) that is paired to a stop codon.  It is reactive, so the cell doesn’t store it.  

Scientists have tried to learn more about the function of this strange amino acid and why it’s found in people and other vertebrates but not other species.

“In previous studies, we discovered that the machinery of selenocysteine had been lost many times in the course of evolution, and we began to take an interest in why it disappears so easily in some groups but not in others,” explained the ICREA ( Catalan Institution for Research and Advanced Studies) Research Professor Toni Gabaldón, head of the Centre for Genomic Regulation’s (CRG) Comparative Genomics group in Barcelona.

In new work reported in Nature Microbiology, researchers focused on fungi, the only kingdom without an identified selenocysteine-carrying species. They mined databases, and after looking through assessments of 1,000 species identified nine with the amino acid.

“It came as a surprise to us because no fungi were believed to have selenocysteine,” said Gabaldón. The fungi that carry this amino acid are part of fungal groups that have gotten little attention; they probably “diverged at an early stage in the evolution of fungi, which means that we will probably find more cases of selenocysteine when more genomes of these groups are sequenced.”

The ancestor of these fungi also carries selenocysteine. Some of the fungi that came after it retained it while others lost it.

“The question that remains to be answered is why it is lost in some organisms whereas in others these genes are essential,” said Gabaldón. “Understanding why selenocysteine is important in fungi and other branches of the tree of life may help us to understand why it is so important to our species and to define what makes selenium essential to human health.”

In the video above Gabaldón discusses why basic biology research is vital to science.

Sources: AAAS/Eurekalert! Via Center for Genomic Regulation, Nature Microbiology

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