Properties of the chromatin repair activity against O6-ethylguanine lesions in DNA. Mechanism of the reaction

Abstract

Chromatin proteins from rat liver contain a repair activity that removes O6-ethylguanine from ethylnitrosourea-treated DNA. This activity does not depend on divalent cations and works in the presence of EDTA, but does depend on the presence of free thiol groups. Thus, it is destroyed by N-ethylmaleimide and is protected by dithiothreitol. The repair activity on single-stranded DNA is only 20% of what it is on double-stranded DNA; its half-life at 35 degrees C is 55 min, but DNA, ethylated or not, affords some protection. The repair reaction is a transethylation from O6-ethylguanine in DNA onto two different cysteine residues contained in acceptor proteins. The reaction can be followed by monitoring the appearance of ethylated proteins or by disappearance of O6-ethylguanine from DNA.