Researchers discovered a microbe that uses two different translations of the DNA code at random. This 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. The study was published in the journal Current Biology

All organisms receive genetic information from their parents who 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 dictates to the cell which sequence of amino acids to join together to form each protein given the underlying sequence in the DNA.

Amino acid

Similarly that "dot dot dot" in Morse code translates as S, so to the genetic code is read in blocks of three bases (codons) to translate to one amino acid. It was originally thought that any given codon always results in the same amino acid just as dot dot dot always means S in morse code. GGA in the DNA.

The group examined an unusual group of yeasts in which some species have evolved an unusual non-universal code. While humans translate the codon CTG as the amino acid leucine, some of the species of yeast instead translate this as the amino acid serine while others translate it as alanine.

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.

Coin-toss Mechanism

This mechanism is physically manifested the team investigated molecules called tRNAs which act as translators that recognize the codons and bring together the amino acids to make a protein chain.

Serine And Leucine

They found that Ascoidea asiatica, is unusual in having two sorts of tRNAs for CTG one which bridges with leucine and one which bridges with serine. So when CTG comes to be translated, it randomly picks one of the two tRNAs and hence randomly picks between serine and leucine.

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.

They looked at how this strange yeast copes with this 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.