All used chemicals were ACS grade or better (biochemical grade). Chemicals were obtained from Sigma, Merck and Gibco. Biochemicals, enzymes and markers were obtained from Sigma, Boehringer Mannheim, New England Biolabs, Pharmacia, Promega and AGS (Angewandte Gentechnologie Systeme). Radiochemicals were obtained from NEN (DuPont, New England Nuclear).