Prevention of G:C pairing in mouse DNA by complete blocking of guanine residues with glyoxal. Availability of cytosine, adenine and thymine for hydrogen bonding with added unmodified polynucleotides.

We have developed conditions for the reaction of single-stranded DNA with glyoxal which permit blocking of essentially all guanine residues. This procedure effectively prevents base pairing involving these guanine residues, yet permits cytosine, thymine and adenine residues in the DNA to pair with added polynucleotides. Modification of DNA with glyoxal greatly reduces intrastrand helical regions, resulting in a very low binding to hydroxyapatite, as compared to unmodified DNA. Annealing of modified DNA with some synthetic polynucleotides of restricted composition (but not others) leads to a significant increase in binding, presumably because mouse DNA has sequences not containing guanine which are capable of hydrogen bonding to the added polynucleotides. This relatively simple procedure may allow isolation and further study of these guanine-free sequences in DNA.