Microbe Breaks 'Universal' DNA Rule by Using Two Different Translations

Scientists have discovered a microbe that uses two different translations of the DNA code at random.

DNA is often referred to as the blueprint of life, however, scientists have for the first time discovered a microbe that uses two different translations of the DNA code at random. 

The unexpected finding breaks what was thought to be a universal rule, since the proteins from this microbe cannot be fully predicted from the DNA sequence. 

All organisms receive genetic information from the parents, which tell the cells how to make proteins — the molecules that do the chemistry in our bodies. This genetic information comprises DNA molecules made up of a sequence of four chemical bases represented by the letters A, T, C and G: the genetic code that dictates to the cell which sequence of amino acids to join together to form each protein given the underlying sequence in the DNA. 

In a similar way that “dot dot dot” in morse-code translates as ‘S’, so too the genetic code is read in blocks of three bases (codons) to translate to one amino acid, writes the researchers. 

It was originally thought that any given codon always results in the same amino acid — just as dot dot dot always mean ‘S’ in morse-code. GGA in the DNA, for example, translates as the amino acid glycine.

However, researchers from the Milner Centre for Evolution at the University of Bath, and the Max-Planck Insitute for Biophysical Chemistry have now described the first, and unexpected exception to his rule in a natural code. 

Their findings are described in the journal Current Biology. 

The researchers examined an unusual group of yeasts in which some species have evolved an unusual non-universal code. While humans (and just about everything else) translate the codon CTG as the amino acid leucine, some of the species of yeast instead translate this as the amino acid serine, whilst others translate it as alanine. 

 

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This is odd enough in itself. But the team was even more surprised to find one species. Ascoidea Asiatica randomly translated this codon as serine or leucine. Every time this codon is translated, the cell tosses a chemical coin: heads for leucine, tails it’s serine. 

“This is the first time we’ve seen this in any species,” said Laurence Hurst, Professor of Evolutionary Genetics and Director of the Milner Centre for Evolution at the University of Bath. 

“We were surprised to find that about 50% of the time that CTG is translated as serine, the remainder of the time is is leucine,” he added. 

“The last rule of genetics codes, that translation is deterministic, has been broken. This makes this genome unique — you cannot work out the proteins if you know the DNA.” 

 

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To understand how this happens — how this coin-toss mechanism is physically manifested — the team investigated molecules called tRNAs, which act as translators that recognise the codons and bring together the amino acids to make a protein chain. 

“We found that Ascoidea Asiatica is unusual in having two sorts of rRNAs for CTG — one which bridges with leucine, and one which bridges with serine,” said Dr. Martin Kollmar of the Max-Planck Institute. 

“So when CTG comes to be translated, it randomly picks one of the two tRNAs and hence randomly picks between serine and leucine,” he added. 

“Swapping a serine for leucine could cause serious problems in a protein as they have quite different properties — serine is often found on the surface of the protein, whereas leucine is hydrophobic and often buried inside the protein,” added Dr. Stephanie Mühlhausen of The Milner Centre. 

“We looked at how this strange yeast copes with randomness and found that A. asiatica has evolved to use the CTG codon very rarely and especially avoids key parts of proteins.” 

The researchers estimate that the random encoding is 100 million years old, but other closely related species evolved to lose this potentially problematic trait. 

“It’s unclear why A. asiatica should have retained this stochastic encoding for so long. Perhaps there are rare occasions when this sort of randomness can be beneficial,” said Dr. Kollmar. 

 


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