All life exists in
a mirror.
More strictly, the biomolecules that make up biological DNA, RNA, and proteins are all "chiral," and their components have two possible mirror shapes, but in each case, life chooses only one
.
At least so far
.
Researchers today say they have made great strides
in exploring the other side of the mirror.
They redesigned a workhorse enzyme that synthesizes RNA to form mirror images, and then they used this enzyme to build all the RNA needed by ribosomes, the cellular machinery
responsible for building proteins.
Other ingredients still need to be added, but once done, mirror ribosomes may produce proteins that, as new drugs and diagnostics, are not easily broken down
in the body.
It also laid the foundation for a more ambitious goal: to create mirror life, a prospect that has captured scientists' imaginations
since Louis Pasteur discovered mirror compounds in 1848.
"This is an important step in reconstructing the dogma of the Center for Molecular Biology in the mirror world," said Stephen Kent, professor emeritus of chemistry at the University of Chicago who was not involved in the work
.
This dogma refers to the standard operating procedure of life: the genetic code is usually DNA transcribed into corresponding RNA sequences and then translated into proteins, which play important chemical roles
in cells.
Each step is a sophisticated molecular machine
composed of proteins or ribosomes of proteins and ribonucleic acids.
Each molecule produces chiral products
.
Chemists have long been able to synthesize DNA, RNA, and proteins
in opposite directions.
But they never put everything together to create a mirror life, and there isn't even enough evidence to prove that this conceit is possible
.
Ting Zhu, a synthetic biologist at Westlake University in Hangzhou, China, has been working on this vision
for years.
In Zhu's view, the first step is to make mirror ribosomes, and the factory can make many other mirror parts
.
This is no small feat
.
A ribosome is a molecular behemoth made up of three large RNA fragments made up of about 2900 nucleotides in total, plus 54 proteins
.
"The most challenging part is making long ribosome RNA," Ting Zhu said
.
Chemists can synthesize about 70 nucleotide long fragments and stitch them together
.
But to make these three much longer ribosomal RNA fragments mirror image, they needed a molecular machine that could process them out—polymerases
.
In 2016, Zhu and his colleagues first attempted the task, synthesizing a mirror version
of a polymerase from a virus.
Polymerase makes mirror RNA, but it is slow and error-prone
.
In the current study, Zhu and his graduate student Yuan Xu set out to synthesize a mirrored version of a so-called "exhausting enzyme" that is used in molecular biology labs around the world to synthesize long strands of RNA, known as T7RNA polymerase
.
It is a huge, 883-amino acid protein that far exceeds the limits
of traditional chemical synthesis.
But analysis of the crystal structure of T7s X-rays showed that the enzyme may have been divided into three parts, each stitched together from a short segment
.
Therefore, they synthesized three parts, one containing 363 amino acids, the second containing 238 amino acids, and the third containing 282 amino acids
.
In solution, these fragments naturally fold into the correct three-dimensional shape and then assemble into a working T7
.
"It's a huge effort to put such a large protein together," says
Jonathan Sczepanski, a chemist at Texas A&M University.
The researchers then let the polymerase work
.
They assembled mirror genes, encoding three long RNA fragments that the team hoped to make; The mirror T7 RNA polymerase then reads the code and transcribes into ribosomal RNA
.
This result provides a tantalizing glimpse
into the power of mirror elements.
The researchers found that mirror RNAs formed by polymerases are far more stable than the normal version produced by regular T7 because they are not affected by naturally occurring RNA "chewing enzymes" that almost inevitably contaminate these experiments and quickly destroy normal RNA
.
Michael Jewett, a chemist and ribosome expert at Northwestern University, said this resistance to degradation "could open the door to novel diagnostics and other applications," including new drugs
.
For example, the team also used their mirror enzymes to make stable RNA sensors, called ribose switches, that can be used to detect molecules associated with disease, and stable, long RNA, which can be used to store digital data
.
Other researchers have shown that mirror versions of short-stranded DNA and RNA, called aptamers, can serve as powerful drug candidates, avoiding degrading enzymes and the immune system, which disrupts most traditional aptamer drug
candidates.
Harnessing this stability more widely is not as simple as making mirror copies of existing drugs, however, such compounds as erroneous chirality will no longer match the chirality
of their intended target in the body.
Instead, researchers may need to sift through a large number of mirror drugs to find effective ones
.
This new work lays the groundwork
for the fabrication of functional mirror ribosomes.
Jewett says this could make it easier for pharmaceutical companies to make mirror acid strings or peptides
.
Because peptides are extracted from 20 amino acid building blocks, not just the four nucleic acids that make up aptamers, they offer greater chemical diversity and potentially better drug
candidates.
Now, Zhu and his team need to make the remnants of the mirror ribosome
.
The three RNA fragments they synthesized accounted for about two-thirds
of the total mass of the ribosome.
What remains are 54 ribosome proteins and some that work in tandem with ribosomes, all of which are smaller and therefore likely easier to synthesize
.
The next question is whether the complete parts box will assemble into ribosomes
.
Even if they do, the resulting molecular machines may still not work, says George Church, a synthetic biologist at Harvard University, who leads one
of the few groups in the world that studies methods of mirroring life.
In order to produce proteins in large quantities, ribosomes must work
in tandem with an additional range of auxiliary proteins.
Church believes that in order to achieve this function in living cells, it is necessary to rewrite the genetic code of the organism so that genetically engineered ribosomes can recognize all of these proteins, specifically the 20 proteins
that transport amino acids to build new ones.
Church's group is working on this issue
.
"It's very challenging," he said
.
But if everything can be combined, researchers and life may eventually be able to enter another chiral world
.
Mirror-image T7 transcription of chirally inverted ribosomal and functional RNAs
