Acid-Loving Archaeon Reads DNA With Enzyme Strikingly Similar To Our Own

Organisms that live in boiling acid read their DNA using enzymes
surprisingly similar to our own, providing insight into the way in which
the
information stored in DNA is unlocked. In this week's issue of PLoS
Biology, research by Dr. Nicola Abrescia and colleagues has shown that the
enzyme that converts DNA into RNA is conserved between simple
single-celled microorganisms called Sulfolobus and more complicated
'higher'
organisms, including human beings, despite a staggering 2 billion year
evolutionary gulf. The paper explores how evolution has shaped our enzyme
to
accomplish more complex functions.



Life on earth is populated by organisms that can be grouped in three
evolutionary domains: Eukarya (humans, plants, animals etc.), Bacteria (E.
coli,
Chlamydia, etc), and Archaea (including Sulfolobus). Many Archaea are
extremophiles; living in high salt, acid or temperature environments.
Transcription, the process of reading DNA to make RNA (which is in turn
translated into proteins by ribosomes) is a fundamental process common to
all
organisms, and is carried out by the enzyme multisubunit RNA polymerase
(RNAP). Eukaryotes have three different RNAPs, whereas Archaea and
Bacteria
have one. Archaea can serve as a wonderful model system because their
simpler RNA polymerase machinery is related to the more complex eukaryotic
RNAP.


To start transcription, the archaeal enzyme requires two accessory
proteins whilst the eukaryotic counterpart needs at least two more. This
increased
complexity prompts two important questions: how did our polymerase evolve
from the ancestral enzyme; and how does Archaea bypass the requirement of
further co-factor proteins?



New work, from a team of researchers from Spain, the UK, and the US, and
led by Dr. Abrescia, investigates the polymerase from the Archaeon
Sulfolobus
shibatae using X-ray crystallography. This reveals the enzyme's
architecture which confirms its close evolutionary relationship with the
eukaryotic
RNAP. The research also identified a subunit novel to Sulfolobus which has
no equivalent in the eukaryotic enzyme. The striking structural
similarities suggest that the ancestral eukaryote used the same enzyme as
the Archaeon, and that modern eukaryotic RNAP evolved by the addition of
bolt-on proteins that regulate eukaryotic-specific processes. From the
location and topology of the newly identified, Archaeon-only subunit, the
scientists have suggested a mechanism by which Archaea do without the
additional cofactors required by eukaryotes for initiating transcription.



The scientists also noted that the complete structure of the archaeal
polymerase illustrates how the ancestral core enzyme was modulated by
addition
of novel subunits, an evolutionary process that has facilitated the
complexity that we see today in Eukarya.



Citation:

"Evolution of complex RNA polymerases: The complete archaeal RNA polymerase structure."
Korkhin Y, Unligil UM, Littlefield O, Nelson PJ, Stuart DI, et al. (2009)

PLoS Biol 7(5): e1000102. doi:10.1371/journal.pbio.1000102


Source
Plos Biology