03.2019 | THE SCIENTIST 49REPRINTED WITH PERMISSION FROM I. ROSENSHINE ET AL.,
SCIENCE
, 245:1387 (1989); ©
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IN THE BEGINNING: A primordial version of RNA might have incorporated
inosine, a derivative of adenosine, in place of guanosine.LET’S GET IT ON: Scanning electron micrographs show mating archaea fused
together with cytoplasmic bridges.EVOLUTIONStrange Beginnings
THE PAPER
S.C. Kim et al., “Inosine, but none of the 8-oxo-purines, is a plausible
component of a primordial version of RNA,” PNAS, 115:13318–23, 2018.Proponents of the RNA world theory argue that life on Earth
originated from a mixture of self-replicating, information-storing
molecules. But while researchers have discovered ways that
RNA’s pyrimidine nucleosides, uridine and cytidine, could have
formed in primordial conditions, they’ve had less success with
the purine nucleosides adenosine and guanosine, casting the
theory into doubt.
Biologist Jack Szostak’s lab at Harvard Medical School
recently set out to test a new hypothesis: that compounds
called 8-oxo-purines could have acted as substitutes for
modern purines in primordial RNA. His team used an adenosine
derivative, inosine, as a control.
Under early-Earth conditions, 8-oxo-purines turned out to
perform poorly—RNA molecules containing them copied slowly
and with low accuracy. But inosine, unexpectedly, served as an
excellent guanosine substitute. “We were really surprised to see
that actually inosine works almost as well as guanosine, and in
some cases, slightly better,” says Szostak. While it’s impossible to
confirm that inosine really was a component of primordial RNA,
“we’re pretty convinced that it could have happened this way.”
By removing the need for a plausible chemical pathway to
generate guanosine under early-Earth conditions, the paper
“goes a long way to suggesting a solution to a long-standing
problem,” says John Sutherland, a chemist at the MRC Laboratory
of Molecular Biology at the University of Cambridge who was
not involved in the work but, like Szostak, is part of the Simons
Collaboration on the Origins of Life.
Researchers now need only to find out how adenosine could
have formed in order to complete the story of how primordial
RNA might have come together. “The value of this work is not
just in what [Szostak’s group] does next,” says Sutherland, “but
what it suggests other people should do next as well.”
—Catherine Off ordMICROBIOLOGYTrading Spacers
THE PAPER
I. Turgeman-Grott et al., “Pervasive acquisition of CRISPR memory
driven by inter-species mating of archaea can limit gene transfer
and infl uence speciation,” Nat Microbiol, 4:177–86, 2019.Naturally occurring CRISPR-Cas systems in bacteria and archaea
carry DNA memories of invasions by viruses or plasmids. These DNA
sequences, called spacers, instruct Cas proteins to destroy the intruders
should they enter the cell again. Curiously, several species of halophilic,
or salt-loving, archaea isolated from water near Israel’s Mediterranean
coast possess spacers matching the DNA of closely related species,
report Te l Aviv University’s Uri Gophna and colleagues.
Archaea can mate by latching together with cytoplasmic bridges
and exposing their genomes to each other to be recombined.
To test whether archaea pick up spacers during mating, the
researchers let two species of halophilic archaea mingle for
24 hours. Chromosomal markers allowed the team to identify
archaeal spawn of two different parent species. Indeed, members
of both species acquired spacers from each other during mating.
“I was surprised to see that mating would really induce acquisition
of spacers so broadly,” says Gophna. In further experiments, the
team found that mating efficiency fell when archaea had spacers
matching their would-be partner of the other species, suggesting
the spacers thwart such pairings.
The study shows that “CRISPR can target not only phages
and plasmids, but also chromosomal DNA from other species,”
says Luciano Marraffi ni, a microbiologist at Rockefeller University
who was not involved with the work. He thinks that in addition to
warding off attacks, CRISPR infl uences evolution by interfering with
mating between archaeal species.
Gophna’s team observed another oddity—that in the genetic
scramble of interspecies mating, archaea sometimes acquired
spacers that instruct their CRISPR systems to pull out chunks of
their own DNA, encoding mostly nonessential functions. Gophna
sees this “semi-random process” as a way that archaea purge their
genome of possibly unhelpful DNA integrated long ago.
—Carolyn Wilke