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Peter J. Unrau, Associate Professor
Department of Molecular Biology and Biochemistry
Simon Fraser University
B.Sc., McMaster University, 1992
Ph.D., M.I.T., 1996
MSFHR Scholar, CIHR New Investigator
Office: SSB 6114
Phone: (778) 782-3448
Email: punrau@sfu.ca
Ribozyme chemistry and the RNA world
One of the fundamental problems facing biology is the question of our origins. Despite the fact that time and the process of evolution have erased much information about early life on Earth, a number of tantalizing clues remain. The presence of RNA at the core of the ribosome, the extensive use of nucleotide cofactors, and the synthesis of DNA and some amino acids from RNA suggest that RNA and not protein was once the dominant biological catalyst. These clues have led to the proposal of an 'RNA world' hypothesis. This hypothesis assumes that RNA is capable of all the reactions required to maintain a 'ribo-metabolism' while at the same time providing a robust mechanism for the evolution of early life through the base pairing properties of nucleic acid.
I am exploring the chemical versatility of catalytic RNAs (ribozymes). The ability of RNA to recognize small substrates and promote difficult chemistry is poorly understood but is required to provide a solid foundation for the RNA world hypothesis. Specifically, I have been examining the ability of RNA to replicate itself, without the help of protein. My previous work has focused on this problem using in vitro selection (where ribozymes are isolated from large pools of random sequences by iterative rounds of selection and amplification). I have selected and evolved ribozymes able to form a nucleotide from a free pyrimidine base and a tethered form of activated ribose. Nucleotide synthesis would have been essential in an RNA world in order to provide a source of nucleotides for polymerization. These monomers would have been the basic substrates for an RNA-templated RNA polymerase responsible for the replication of a primitive ribo-organism's genome.
Current research interests and opportunities:
We are interested in testing the catalytic properties of RNA. My laboratory will focus on RNA-catalyzed reactions that would have been essential in order to replicate RNA. Immediate goals include:
- In vitro selection of ribozymes able to perform nucleotide synthesis. Can the pyrimidine nucleotide synthase ribozymes previously selected be evolved to allow purine and pyridine nucleotide synthesis? Can any of these ribozymes be engineered to work completely in trans and perform multiple turnover chemistry?
- Selection of ribozymes able to capture chemical energy. How could an ancient metabolism have been powered? One possibility, exploited by modern metabolism, is the oxidation of aldehydes to capture energy in the form of acyl-phosphate compounds. Can a ribozyme sequence be isolated that performs similar chemistry? Finding such a ribozyme would demonstrate that RNA-based life could have harvested both reducing power and activated carbon for metabolism, using a realistic abiotic source.
- Development of new in vitro selection technologies. How can current in vitro selection methods be improved? A successful in vitro selection must isolate catalytic sequences from a huge background of inactive sequences. In practice, a substrate for the ribozyme reaction is tethered to the 3' or 5' end of the RNA pool molecules. Active sequences able to react with a second tagged substrate can be purified away from the inactive molecules in the pool and enriched. This approach has two drawbacks. First, tethering one of the substrates to the ribozyme makes it difficult to engineer true multiple turnover catalysts. Second, requiring the active site of the ribozyme to form at either the 3' or 5' end of the nucleic acid sequence places a heavy combinatorial constraint on the number of active sequences available in a random sequence pool. I would like to overcome these problems and significantly extend the power of in vitro selection.
Last updated 02/08/2008