On the use of ultraviolet light to study protein-DNA crosslinking

Irradiation of Ehrlich ascites chromatin with ultraviolet light (u.v.) leads to protein-DNA crosslinking as determined by CsCl isopycnic ultra-centrifugation and SDS-polyacrylamide gel electrophoresis. At the most 4.5% of the chromatin proteins labelled with (14C)-lysine and (14C)-arginine can be crosslinked to DNA at u.v. doses between 3.6 X 10(4) to 10.8 X 10(4) ergs/mm2. We find however that the crosslinking reaction is hindered by protein breakage induced by u.v. light of wave length of less than 2950 A. Our results point out that caution must be used in the interpretation of studies on protein-nucleic acid interactions using u.v. light.