Opposite staining effect of two silver-staining techniques on sister chromatids

Opposite differential staining between sister chromatids was obtained by two silver-staining techniques on chromosomes replicated twice in medium containing 5-bromodeoxyuridine (BrdU) and pretreated with Hoechst plus black light. Both silver-nitrate and silver-carbonate staining were affected by chemical extraction and enzyme digestion of chromosomal proteins. Prestaining of silver nitrate or silver carbonate also blocked the fluorescences of protein dyes. However, removal of chromosomal DNA affected the silver-carbonate but not the silver-nitrate staining; the fluorescences of DNA dyes were blocked by the prestaining of silver carbonate but not silver nitrate. Chromosomal protein labelling was released only slightly and its relative amount between BrdU bifilarly substituted and unifilarly substituted chromatids was unchanged during pretreatment of Hoechst plus black light. We speculate that chromosomal non-histones are the targets for silver-nitrate stain, and DNA-non-histone complexes for silver-carbonate stain.