Ribozyme catalysis

Polyamine/Nucleotide Coacervates Provide Strong Compartmentalization of Mg²⁺, Nucleotides, and RNA

Phase separation of aqueous solutions containing polyelectrolytes can lead to formation of dense, solute-rich liquid droplets referred to as coacervates, surrounded by a dilute continuous phase of much larger volume. This type of liquid-liquid phase separation is thought to help explain the appearance of polyelectrolyte-rich intracellular droplets in the cytoplasm and nucleoplasm of extant biological cells and may be relevant to protocellular compartmentalization of nucleic acids on the early Earth. Here we describe complex coacervates formed upon mixing the polycation poly(allylamine) (PAH, 15 kDa) with the anionic nucleotides adenosine 5'-mono-, di-, and triphosphate (AMP, ADP, and ATP). Droplet formation was observed over a wide range of pH and MgCl2 concentrations. The nucleotides themselves as well as Mg(2+) and RNA oligonucleotides were all extremely concentrated within the coacervates. Nucleotides present at just 2.5 mM in bulk solution had concentrations greater than 1 M inside the coacervate droplets. A solution with a total Mg(2+) concentration of 10 mM had 1-5 M Mg(2+) in the coacervates, and RNA random sequence (N54) partitioned ∼10,000-fold into the coacervates. Coacervate droplets are thus rich in nucleotides, Mg(2+), and RNA, providing a medium favorable for generating functional RNAs. Compartmentalization of nucleotides at high concentrations could have facilitated their polymerization to form oligonucleotides, which preferentially accumulate in the droplets. Locally high Mg(2+) concentrations could have aided folding and catalysis in an RNA world, making coacervate droplets an appealing platform for exploring protocellular environments.

In vitro evolution of a self-alkylating ribozyme

RNA enzymes are postulated to have catalysed all chemical reactions in the earliest living cells. This idea is now investigated in a search for alkyl transferases from a pool of random sequence RNAs. Selection for self-biotinylation yields a transfer RNA-like ribozyme that efficiently catalyses carbon-nitrogen bond formation. Ribozymes can thus promote reactions other than those involving the RNA sugar-phosphate backbone, suggesting that RNA may be capable of a broad range of catalytic activities.

Sequence-selective recognition and cleavage of double-helical DNA

Single sites within long double-helical DNA molecules can be recognized by a variety of mechanisms. Different strategies have been used to adapt sequence-specific recognition to sequence-specific cleavage of duplex DNA. Any nucleic acid can be converted into an artificial nuclease by the attachment of a cleaving reagent. Alternatively, a sequence-specific ligand can be used to protect a methylase recognition site from methylation. The protected site may then be cleaved selectively by a restriction endonuclease (the so-called 'Achilles heel' cleavage technique). Recent developments in this area have shown that it is possible to cleave chromosomal DNA at single sites within bacterial and eukaryotic genomes.