Chemical restriction: strand cleavage by ammonia treatment at 8-oxoguanine yields biologically active DNA

Cleavage of DNA single and double strands at an 8-oxoguanine-containing nucleotide occurs in 90 % yield if the modified oligonucleotide is treated with NH(3) and O(2) at 60 degrees C. The mechanism of this oxidative cleavage reaction was studied, and the reaction was applied to the generation of single-stranded overhangs on PCR-amplified DNA that can be ligated. As an example, the lac Z' gene was amplified by PCR with 8-oxoguanine modified primers, restricted by ammonia treatment, ligated into a plasmid vector, transformed in Escherischia coli cells, and screened for blue colonies. This method guarantees efficiencies comparable to the standard cloning procedure with restriction enzymes, and it allows the design of any 3'-overhang independent of the sequence of the cloned DNA.     

Polymerase Synthesis and Restriction Enzyme Cleavage of DNA Containing 7‐Substituted 7‐Deazaguanine Nucleobases

Previous studies of polymerase synthesis of base‐modified DNAs and their cleavage by restriction enzymes have mostly related only to 5‐substituted pyrimidine and 7‐substituted 7‐deazaadenine nucleotides. Here we report the synthesis of a series of 7‐substituted 7‐deazaguanine 2′‐deoxyribonucleoside 5′‐O‐triphosphates (dG^RTPs), their use as substrates for polymerase synthesis of modified DNA and the influence of the modification on their cleavage by type II restriction endonucleases (REs). The dG^RTPs were generally good substrates for polymerases but the PCR products could not be visualised on agarose gels by intercalator staining, due to fluorescence quenching. The presence of 7‐substituted 7‐deazaguanine residues in recognition sequences of REs in most cases completely blocked the cleavage.     

Cover Picture: Selective Protein Degradation by Ligand‐Targeted Enzymes: Towards the Creation of Catalytic Antagonists (ChemBioChem 6/2003)

The cover picture shows a ligand‐targeted proteinase enzyme or “catalytic antagonist” acting as a molecular angler fish: By precisely positioning different binding ligands (L) around the active site “mouth” of a degradative proteinase enzyme, target proteins (TP) can be plucked from solution, locked in position adjacent to the catalytic triad “jaws”, and in this way readily and specifically degraded. The hunting strategy of the deep sea angler fish, which uses a lure above its mouth, illustrates this principle. Further details can be found in the article by B. Davis, R. R. Bott, J. B. Jones et. al. on pp. 533–537.