Iimagine a universe in which screws are tightened by turning counter-clockwise. In this inverted world, bolts and nuts would be mirror images of those we use most often. Reflections in every way similar but not identical, because they are incompatible with each other: impossible, in fact, to screw a bolt from the “left” universe into a nut from the “right” universe.
At the molecular level, the situation is similar for the main biological components that make up our cells and ensure their functioning. The molecular backbone of DNA and RNA, for example, is made up of straight molecular shaped sugars. A consequence of this asymmetry is that the DNA helix threads twist to the right. Similarly, proteins are also oriented.
Could we design a mirror biological system, in which DNA, RNA and proteins would have the opposite orientation to that in which the whole living world is engaged? This is theoretically possible, since we know how to artificially synthesize the basic compounds necessary both in their right and left forms, and assemble them in chains of a few tens to hundreds of units. The practical difficulty is to orchestrate these molecules to reconstitute their biological functions: the DNA must be copied identically to perpetuate itself, which requires proteins called DNA polymerases; then, transcribed into messenger RNA, by an RNA polymerase; and, finally, messenger RNA is read and translated by ribosomes (made up of several RNA molecules, and more than fifty proteins), assisted by transfer RNA, to make proteins. The main constraint is the thread: as with incompatible bolts and nuts, how can the left DNA be copied if only DNA polymerases capable of reading the right DNA are available? How to get mirror polymerases, if you only have right messenger RNAs, right ribosomes, etc. ?
Small RNA as probes
This threading problem implies being able, at least to start the machine, to produce all the mirror constituents artificially. A feat that the laboratory directed by Ting Zhu at Westlake University (Hangzhou, China) is on the way to achieving. In a previous studythe team had synthesized in vitro a fairly long left-hand DNA as well as the mirror DNA polymerase capable of copying it.
In a new, even more impressive article, a doctoral student, Yuan Xu, addressed the case of RNA. She first synthesized a mirror RNA polymerase – a complex task, due to the very large size of this protein, which she was able to carry out by breaking down the polymerase into several cooperating modules. This allowed him to transcribe left-hand RNA molecules of different functions from left-hand DNA: ribosomal RNAs, transfer RNAs, and small probe-function RNAs that fluoresce when they bind to specific compounds. The next challenge will be to synthesize all the mirror ribosomal proteins, in order to reconstitute mirror ribosomes capable of translating left messenger RNAs into mirror proteins.
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