The role of chromosomal proteins in the induction of a differential staining of sister chromatids by light.

Fixed chromosomes of human lymphocytes, cultured in the presence of bromodeoxyuridine (BrdU) during two cell cycles, were exposed to near-ultraviolet irradiation, stained with Giemsa, and after destaining, were subjected to either Coomassie Blue or Feulgen--Schiff staining. A differential reaction of sister chromatids was first revealed by Coomassie Blue staining. Differential staining with Giemsa required a longer irradiation time. This appeared to be reduced after the addition of dithiodipyridine to the cells during their last few hours of culture. The differential pattern obtained after Coomassie Blue staining was the inverse of that obtained after Giemsa staining. From these findings we concluded that the induction of sister chromatid differentiation by light in BrdU-substituted DNA containing chromosomes occurs primarily via chromosomal proteins, presumably by differential breakage of their disulphide bonds. The results of the Feulgen--Schiff staining indicated that differential depurination of BrdU-containing DNA could occur, although only after very prolonged irradiation. A faint though distinctly differential Feulgen--Schiff pattern of sister chromatid staining, resulting from differential removal of DNA, was observed after photosensitization by specific DNA-binding dyes. Thus, DNA seems to be affected only under more extreme conditions.