UV light-induced duplex-to-duplex crosslinking of DNA molecules in aqueous ethanol solutions

Ultraviolet light is known to generate crosslinks between the complementary strands of DNA and between DNA and proteins. Here we demonstrate that the UV light also crosslinks DNA duplexes to other DNA duplexes. However, the duplex-to-duplex crosslinks only appear in the presence of about 75% (vol/vol) ethanol plus a millimolar or submillimolar concentration of monovalent or divalent cations, e.g. 2 mM Na+. Methanol or formamide are ineffective. The present observations provide a direct means to detect physical contacts of DNA molecules or their parts, e.g. during recombination. It is remarkable that the solution conditions leading to the duplex-to-duplex UV light-induced crosslink formation are the same as those inducing the B-to-A conformational transition of DNA.