Mirror, mirror…
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The 19th century French biologist and chemist Louis Pasteur once wrote, “The universe is asymmetric and I am persuaded that life, as it is known to us, is a direct result of the asymmetry of the universe or of its indirect consequences.”
On Earth, life is indeed asymmetric, right down to its very molecules. And in the case of DNA, it’s right handed (sorry lefties!). If you look at natural DNA from below and draw an arrow up the structure, following its characteristic double helix, you will find that the arrow always turns clockwise - to the right. This handedness is called chirality. DNA is not the only chiral biological molecule, in fact nucleotides, amino acids and many other naturally occurring organic compounds are chiral too.
The mirror image of DNA, which turns to the left, is called L-DNA. L-DNA has gained some attention in recent years because it has the potential to be used in data storage. Unlike naturally occurring DNA, L-DNA can’t be recognised by the naturally-occurring enzymes that break down DNA, making it a good option for robust data storage. But there’s a catch - making L-DNA is difficult because naturally occurring DNA polymerase enzymes only make right handed DNA.
Over the past few years, scientists from Tsinghua University in Beijing, China, have flipped the problem around, creating mirror-image DNA polymerases, made with mirror-image amino acids, which can build L-DNA in the same way natural polymerases build right handed DNA.
The researchers made their first flipped polymerase back in 2016. Because they had to build from scratch, they used the smallest polymerase they could find - an enzyme from a pig virus made up of just 174 amino acids. Although the mirror-image enzyme worked and created L-DNA, it was very slow and made lots of mistakes.
But to make L-DNA data storage a reality, the researchers knew they needed a better polymerase enzyme that could make L-DNA synthesis as fast and efficient as its right handed cousin. So they set about making a southpaw version of a DNA polymerase called Pfu, which is stable, efficient and accurate. Unfortunately, it’s also huge and made up of a whopping 775-amino acids.
With no enzymes or bacteria to help, making a protein of this size is a gigantic task. The scientists split the protein into smaller sections which they made and then joined up together to make the full mirror-image enzyme. When they were finished, their mirror polymerase enzyme became the largest chemically synthesised protein to date.
Importantly, it’s not just enough to be able to write left-handed DNA to store data - you need to be able to read the sequence to extract the information back out again. Fortunately, in 2018, the team also developed a way to sequence L-DNA using an adapted version of Maxam-Gilbert sequencing, opening the door to using L-DNA as a data storage molecule (For more about the Maxam-Gilbert method, check out our recent episode on the history of DNA sequencing, Reading the book of life).
The Chinese team have so far used their mirrored enzyme to encode a passage of text written by Louis Pasteur in 1860, where he speculated about mirror biology: “And, consequently, if the mysterious influence to which the asymmetry of natural products is due should change its sense or direction, the constitutive elements of all living beings would assume the opposite symmetry. Perhaps a new world would present itself to our view. Who could foresee the organisation of living things if cellulose, right as it is, became left? If the albumen of the blood, now left, became right? These are mysteries which furnish much work for the future, and demand henceforth the most serious consideration from science.”
To prove the robustness and storage potential of L-DNA, the researchers stored their coded message in pond water for a year before successfully amplifying and sequencing it to decode the message. The same sequence encoded in natural DNA didn’t even last a day under the same conditions.
The team have also used their mirrored polymerase to encode an entire mirrored gene. The gene, which is around 1500 bases long, encoded part of a mirror-image ribosome which the researchers hope to create in future as a part of their reverse molecular toolkit on their way to an entire mirror-image cell. More than 160 years after Pasteur speculated on what might happen if the constitutive elements of life were to be reversed, we could be about to find out.
References
Mirror-image polymerase makes mirror gene and more, Chemical & Engineering News
Mirror-Image Enzyme Copies Looking-Glass DNA, Scientific American
Sequencing Mirror-Image DNA Chemically, Science Direct
