The central dogma of molecular biology is that DNA makes RNA makes protein. This relies on a specific underlying code which relates given triplets of RNA nucleotides into specific amino acids. Each of the 20 amino acids is represented by one or more RNA triplets, or codons: UAC is decoded as tyrosine, for example, and UGC as cysteine. (U is the RNA nucleotide containing uracil, A is adenine, C is cytosine, and G is guanine.) For some time the code had been thought to be the same in all organisms. But exceptions have been seen before, particularly in mitochondria.
In a new study published online this week in the open-access journal PLoS Biology, Federico Abascal, Rafael Zardoya, and colleagues show that in the mitochondria of arthropod there are two nonstandard codes, and suggest that genetic code changes within a lineage may be more frequent than was earlier believed.
The authors aligned the mitochondrial coding sequence from >600 animal species looking for conserved codons and identifying which amino acid (AA) it specified in the corresponding protein. The most frequent AA was taken to be the canonical translation of that codon. What they found was that although most codons adhered to the common genetic code in all species, there was nonetheless a surprising trend in the arthropods, the largest of all animal phyla. Typically, AGG translates as the amino acid serine. However, among the arthropod mitochondrial genomes, AGG coded for serine in some species and lysine in others. The authors' analysis of the patterns of change also suggests that the original arthropod mitochondrion used AGG for lysine, not serine.
The observed variety suggests the code has changed multiple times between the two genetic codes. It might be that pairing of AGG and lysine is disadvantageous for the organism employing it, so that loss or reversion over time would be favored. This might also suggest the existence of multiple other nonstandard codes within other lineages. Who knows what other alternatives might be decoded with this method in the future.
Citation: Abascal F, Posada D, Knight RD, Zardoya R (2006) Parallel evolution of the genetic code in arthropod mitochondrial genomes. PLoS Biol 4(5): e127.
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