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Tiny One-Celled Microbe Breaks DNA Rule All Other Life Forms Follow

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An unusual species of microbe has been found to be the first organism to break what scientists call the most fundamental rule of genetics.

Scientists from the Milner Centre for Evolution at the University of Bath and the Max-Planck Institute for Biophysical Chemistry in Gottingen have discovered a rare group of yeasts that seems to be the only exception to a universal DNA rule: that proteins are always translated from the same genetic codes.

In new a new paper published in the journal Current Biology, the researchers explain how this particular yeast group disregards this universal biological rule and translates its own proteins randomly.

Universal Rule Of DNA

All organisms have genetic information passed on by their parents to allow them to develop and function properly. This information is stored in DNA, which carries it as a code in the form of four chemical bases: adenine (A), thymine (T), cytosine (C), and guanine (G).

The code is read as a set of three letters representing the bases. This is called a codon. Each codon translates to an amino acid, the basic building block of protein. A series of codons dictates how the organism forms proteins that are necessary to survive.

Through decades of research, scientists have come across one pattern so ubiquitous that they have come to call it a rule. A codon always translates to one amino acid. For instance, TTT always translates to the amino acid phenylalanine and AAA always translates to lysine.

Different codons can translate to the same amino acid, as TTC can also code to phenylalanine and AAG to lysine. However, no two amino acids have the same codons.

An Exception To The Rule

An unusual yeast has been found to be the only organism that blatantly disregards this rule. The researchers have found that the yeast carries a genetic code that does not follow the universal codes used by all other organisms.

In other life forms, CTG translates to leucine. For this yeast, however, CTG is sometimes serine in some species and alanine in others.

What is even more astonishing to the researchers is that one particular species, Ascoiedea asiatica, randomly translates CTG to serine, like other species in its group. Sometimes, it also translates to leucine, like humans and other organisms.

This is the first time scientists have found a species that does this. Even if they have the genetic code of the species, they will still not be able to figure out the proteins.

"We were surprised to find that about 50 percent of the time that CTG is translated as serine, the remainder of the time it is leucine," says Laurence Hurst, professor of evolutionary genetics at the Milner Centre for Evolution.

How It Happens

To crack the code of the mysterious rule breaking, the researchers looked into A. asiatica's tRNA, the molecules responsible for reading the codons and translating them into amino acids.

What the team found was A. asiatica has two kinds of tRNA for CTG. One of them translates to leucine and another to serine.

Unfortunately, the random translation could pose problems for the species. Leucine and serine are two very different amino acids. Leucine repels water and tends to burrow into the center of the protein, while serine is often found on the surface.

To get around this problem, the yeast seems to have evolved so that it does not use CTG often. In cases where it does, it only uses it in parts of the protein that are not key for survival.

The researchers trace this trait as far back as 100 million years old. Not surprisingly, other yeast species have evolved out of this particular translation. Scientists have yet to find a reason why A. asiatica has kept this trait for so long.

"Perhaps there are rare occasions when this sort of randomness can be beneficial," says Martin Kollmar of the Max-Planck Institute for Biophysical Chemistry.

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