As science has delved into the DNA of thousands of species, one pattern was so consistent, and apparently so necessary, it came to be regarded as a rule. A specific set of genetic instructions always translates to the same protein. Now, however, one species has been found that instead translates its code in two different ways, with no rhyme or reason as to which will be used.
DNA contains four chemical bases represented by the letters A, T, C, and G. Genes are made up of sequences of these bases, and it is these sequences that determine how amino acids will be joined together to make proteins. Three letters in succession correspond to a specific amino acid. There can be several different sets of letters that all code for the same molecule, but you can't have different molecules coded by the same set of letters. (As a slight complication, some species use certain letters as punctuation that code for molecules in others).
Irrespective of the plant, animal, or bacteria in which DNA may exist, the sequence CTG translates to leucine, for example. Or so it seemed, until the discovery that some yeasts instead translate CTG to serine and still others to alanine.
While that finding surprised molecular biologists in the 1980s, a team including Professor Laurence Hurst of the University of Bath have now found something considerably more shocking. For the microbe Ascoidea asiatica, CTG sometimes translates as leucine (like most of the living world) and sometimes as serine (like a few other yeasts). Which amino acid will be coded for is a fundamentally random process, not dependent on the context of neighboring sequences.
"The last rule of genetics codes, that translation is deterministic, has been broken,” Hurst said in a statement. “This makes this genome unique – you cannot work out the proteins if you know the DNA."
Cells use tRNAs to translate DNAs into proteins. These use the same four bases, except uracil replaces thymine. Dr Martin Kollmar of the Max-Planck Institute of Biophysical Chemistry, a co-author with Hurst of the Current Biology paper reporting the discovery, said: "We 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.” The selection of which tRNA will do the translating is random.
For a species to develop its own code is strange, but it is at least possible to see how it could live. The organism does not care that every other living thing uses something different. What is much more puzzling is how an organism can operate in such a random way. It is not as if leucine and serine have such similar properties as to be interchangeable – one is hydrophobic while the other is not and they usually don't even occupy the same part of a protein. Despite this, A. asiatica has a growth rate similar to relatives with more normal biology.
The authors report that A. asiatica doesn't use CTG often, and only in less essential proteins, but that still leaves open the question of why this happens at all.
Some of A. asiatica's relatives also produce two tRNAs for CTG, but one of these are non-functional. The extra tRNA, therefore, probably appeared more than 100 million years ago, and most species have evolved around it, while A. asiatica has survived all this time despite the apparent hindrance. The authors speculate there may be some circumstances where this astonishing trait is a benefit, although they don't know what this would be.