Expanding the genetic code with quadruplet codons
Synthetic biology advance furthers work on engineered proteins for
medicinal and other uses

Date:
September 29, 2021
Source:
Scripps Research Institute
Summary:
Cells working with an expanded genetic code could make more
diverse medicines. A new study shows scientists are within striking
distance.



FULL STORY ==========================================================================
One of modern biologists' most ambitious goals is to learn how to expand
or otherwise modify the genetic code of life on Earth, in order to make
new, artificial life forms. Part of the motivation for this "synthetic
biology" research is to understand more about the evolution and the logic
of the natural biology we've inherited. But there's also a very practical motivation: Cells can be used as efficient factories for making a broad
array of useful molecules -- especially protein-based therapeutics,
which account for an increasing share of new medicines. Cells working
with an expanded genetic code could make a much more diverse set of such medicines and could do so in a way that greatly simplifies the overall
process of developing and manufacturing them.


==========================================================================
The realization of the grand goal of a working, useful synthetic biology
is still some years off. But in a study published this week in Nature Communications, scientists have taken a significant step closer to it,
by developing and demonstrating key components of an expanded genetic
code system.

"We've supplemented the synthetic biology toolkit to streamline
investigations into genetic code expansion," says study senior author
Ahmed Badran, PhD, an assistant professor in the Department of Chemistry
at Scripps Research.

The natural genetic code underlying life on Earth is used by cells to
translate information contained in DNA and RNA into the amino-acid
building blocks of proteins. DNA and RNA molecules are chain-like
molecules that encode information using an "alphabet" of four nucleotide building blocks, or "letters." Molecules called transfer RNAs (tRNAs)
decode this information by recognizing three letters at a time,
translating each three-letter "codon" into a single amino-acid building
block of a protein. This triplet codon system in principle can encode
64 different amino acids (43) -- yet typically only 20 amino acids are
used in most organisms.

By contrast, the envisioned quadruplet system, based on four-letter
codons, could encode 256 (44) distinct amino acids. Obviously, most of
those would not exist in natural proteins, although some could be slight variations on natural amino acids, enabling proteins to be made with
much more finely tuned characteristics, for example to optimize their effectiveness and safety as medicines.

The enormous challenge here comes from the fact that the system of
gene-to- protein translation is a complex one in which multiple components
must work together smoothly. The system that exists in living organisms
on Earth presumably took many millions of years to evolve to its present
levels of accuracy and efficiency. Prior efforts to engineer whole new
systems, including quadruplet-codon systems, have shown some promise in
recent years.

In the new study, Badran and his team used an evolutionary,
survival-of-the- fittest, technique called directed evolution to evolve
a small set of tRNAs that in principle could work in a quadruplet
system. The scientists showed that these quadruplet tRNAs could be used
to translate segments of a protein within bacterial cells. They were able
to translate six identical quadruplet codons after one another, and even translate four very different quadruplet codons in the same protein -- and could do so at efficiencies that are for the first time within striking distance of what would be needed for a functional quadruplet system.

Badran emphasizes that although a quadruplet code system is still very
much in the early, methods-development stage, it should be very useful
if it can be made to work -- especially in enabling the straightforward synthesis of proteins with "non-canonical" amino acids that aren't found naturally in proteins. Such ncAAs, as they are called, could be used to
give proteins novel biological properties, including the provision of convenient, safe "handles" on a protein -- for the placing of chemical modifications to improve the protein's therapeutic properties, for
example, or for the attachment of a toxic "warhead" on a tumor-homing
cancer drug.

"One could theoretically program a sequence of DNA that would be
translated, in a living cell, into a protein that contains a complex set
of modifications - - modifications that otherwise would be difficult or impossible to add," Badran says.

Badran, who joined Scripps Research earlier this year, worked at the
Broad Institute of MIT and Harvard during the study.

In addition to Badran, the study, "Multiplex suppression of four
quadruplet codons via tRNA directed evolution," was co-authored by Erika DeBenedictis and Gavriela Carver, of the Broad Institute, and Christina
Chung and Dieter So"ll of Yale University.

========================================================================== Story Source: Materials provided by Scripps_Research_Institute. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Erika A. DeBenedictis, Gavriela D. Carver, Christina Z. Chung,
Dieter
So"ll, Ahmed H. Badran. Multiplex suppression of four quadruplet
codons via tRNA directed evolution. Nature Communications, 2021;
12 (1) DOI: 10.1038/s41467-021-25948-y ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/09/210929163758.htm

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