The artificial life research tide may be shifting at last, from dry robots and clever software back to wet chemistry. Gerald Joyce and Jonathan Sczepanski at the Scripps Research Institute in La Jolla, California have created a chemical—an RNA enzyme—that can copy RNA in starter environments containing right- and left-handed starter chemicals. (reported in Nature Nov. 20.)
The key word in the report of this accomplishment is and. The biochemical reactions of living cells on Earth now are very fussy. Only right-handed DNA and RNA molecules are found on Earth these days. They don’t react with chemicals of the wrong handedness. However, “pre-biological” building blocks for life on the very early Earth were both right-and left-handed. So what happened to turn them all right?
The RNA-copying molecules (ribozymes) Joyce and Sczepanski designed are also more efficient and more flexible than other RNA-type enzymes. If such an enzyme existed on the young Earth, it could have created a
complex molecule like RNA. Then, it could “break the mirror” (favor right-handedness) by selecting right-handed amino acids and nucleic acids (RNA building blocks), thus giving them a selective advantage.
Joyce and Sczepanski created the new ribozyme by isolating those that could copy RNA of the opposite handedness. It was amazingly easy, which speaks well for its evolutionary usefulness. And even now their talented
ribozymes are producing an increasing range of RNA sequences.
Such ribozymes would have done well on the early Earth, building RNA sequences with right and left amino acids and nucleic acids. They can do this because their ribozyme binds to its target building blocks by noting the molecules’ shapes, not their sequences as modern RNA does. Selection pressure (“competitive advantage”) could then do its thing to leave us with only right-handed RNA nowadays.
It will be interesting to see if George Church at Harvard can use the new non-fussy ribozyme to make left- and right-handed living cells. Michael Kay at the University of Utah is working to build mirror image RNAs that can put together mirror building blocks.
The next step is to design tools that can detect possible mirror artificial life (alife) on other planets and their moons. Meanwhile, creating alife from scratch seems a lot more doable now, for S and J’s new ribozyme is close to being alive, with its ability to replicate and evolve.
(Information summarized from www.quantamagazine.org/20141126-why-rna-is-right-handed/ by Emily Singer, November 26, 2014)