Transcript Slide 1

RNA World
Weaker hypothesis:
There was a stage of evolution at when RNA molecules performed
both genetic and catalytic roles. DNA later took over the genetic role
and proteins took over the catalytic role.
Almost certainly true
Stronger hypothesis:
The RNA world arose de novo in the form of self replicating
ribozymes.
The jury is still out
Lectures based on:
Joyce (2004) The antiquity of RNA-based evolution Nature 418: 214-221
Orgel (2004) Prebiotic chemistry and the origin of the RNA world. Crit. Rev.
Biochem. Mol. Biol. 33: 99-123
Jeffares, Poole & Penny (1998) Relics from the RNA world. J. Mol. Evol. 46:
18-36.
RNA world idea originated in 60’s as a theoretical solution to the
chicken and egg problem of DNA and proteins.
Self-splicing introns. First RNA catalysts to be discovered. Tom
Cech (1982).
‘RNA World’ term coined by Walter Gilbert (1986).
Example of an RNA catalyst
Hammerhead ribozyme
Cleaves RNA at a specific
point.
Rolling circle mechanism
of replication of virus-like
RNAs in plants. Chops
long strand into pieces.
Chemistry required for RNA formation
Prebiotic synthesis of sugars - Formose reaction – Butlerow (1861)
Formaldehyde  Sugars (in aqueous solution)
2 CH2O
+
CH2OHCHO  2 CH2OHCHO
formaldehyde glycoaldehyde
Glycoaldehyde catalyses its own formation – autocatalytic
Glycoaldehyde reacts to form a mixture of tetrose, pentose
and hexose sugars.
Oro (1950s) showed plausible prebiotic synthesis of purines from HCN
Many steps involved
Other steps in prebiotic synthesis
Pyrimidines also
ribose + base  nucleoside (weakest link)
phosphorylation of nucleosides  nucleotides
Conclusion from Orgel (2004)
“no convincing prebiotic total synthesis of nucleotides”
“many individual steps have been demonstrated”
“few reactions give high yields”
“complex mixtures of products”
Clutter of RNA synthesis (Joyce)
Why is this particular set of monomers used for
nucleic acids?
How is this set synthesized specifically?
Where is the chemistry occurring? Earth, or space? Hydrothermal vents?
Polymerization of nucleotides.
Must be ‘activated’ - polymerization is
uphill thermodynamically.
+

+
In vivo synthesis of RNA works like this – from nucleoside triphosphates.
This found to be too slow in lab. Other activating groups were used.
Oligomers up to 16mers formed using Pb++ catalyst.
Up to 40mers formed using Montmorillonite clay catalyst (Ferris)
Don’t know how monomers were activated prebiotically.
Template directed synthesis of
complementary strand possible
from many oligomers.
An RNA organism must have had
a metabolism.
Hypothetical pathway for RNA
catalyzed RNA synthesis (Joyce)
Synthesis of nucleosides
Phosphorylation
Generation of NTPs
Creation of activated nucleotides
Stepwise polymerization
Can we synthesize a replicase ribozyme?
Johnston et al. (2001)
RNA-dependent RNA polymerase.
General – should work on any sequence
In vitro selection from large pool of random
sequences
Black – ribozyme
Red – template
Orange – primer
Blue – randomized part
In A, template is paired to ribozyme
In C, it is not. Blue domain recognizes nonspecific features of the template/primer.
Templates extended by up to 14 extra nucleotides. Limited by ribozyme
stability in solution. Fidelity 96.7% (see error threshold theory)
Are there alternatives to RNA?
RNA
a – Threose Nucleic Acid – TNA
b – Peptide nucleic acid – PNA
c – Glycerol derived nucleic acid
d – Pyranosyl RNA
RNA hybridizes with other nucleic acids. Information is not lost.
DNA-RNA hybrids  DNA takes over at end of RNA world.
Maybe TNA or PNA preceded the RNA world. Information passed to RNA.
Would need to show that the alternative was easier to synthesize than RNA.
Two scenarios from
Segré & Lancet (2000)
A – RNA first (strong RNA
world hypothesis)
B – Lipids first (lipid world
hypothesis –
compositional genomes –
metabolism without
genes)
When was the RNA world?
Dating of
rocks and
meteorites
What preceded RNA?
Another polymer?
Metabolism only?
Last oceanvaporizing impact.
Lunar craters
Microfossil evidence (?)
Stromatolites.
Phylogenetic methods
(divergence after LUCA)
Isotopic evidence
for life (?)
Why do we believe there was an RNA world (weaker hypothesis) ?
Translation depends on RNA:
mRNA supplies the information for protein synthesis.
Active ingredient of the ribosome is rRNA – 3d structures
show site of peptidyl transferase reaction. Proteins probably
added as a late addition to the ribosome.
tRNAs also essential for translation.
rRNA and tRNA must be relics of the RNA world.
Which other RNAs might be relics? Jeffares (1998)
• catalytic RNA more likely to be ancient than information-carrying RNA
(newly evolved metabolic functions would be proteins)
• ubiquitous (all domains of life – prior to LUCA)
• central to metabolism
Tendency: RNA  RNP (ribonulceoprotein)  protein
In early RNP particles, proteins act as cofactors to improve RNA
catalysts. Eventually proteins replace RNA in most functions.
Need genetic code to specify synthesis of long proteins
Probable relic RNAs:
Ribonuclease P – RNaseP (is a RNP) – tRNA processing
snoRNAs – small nucleolar RNAs – rRNA processing (methylation of
bases in specific positions on rRNA)
snRNAs – small nuclear RNAs – mRNA processing in eukaryotes spliceosome – U1, U2, U5 etc.
several other possible candidates.
Estimated genome of the last riboorganism – at least 10-15 kbp
probably double stranded RNA genome (i.e. DNA came last, after RNA
and proteins). Possibly several copies each of small linear chromosomes.