Ultrastructural localization of DNA on ultrathin sections of resin-embedded tissues by the lactoferrin-gold complex

Lactoferrin, a DNA-binding protein, was complexed to colloidal gold and applied on ultrathin sections of resin-embedded plant tissues and bacterial cultures. Optimal results were obtained when lactoferrin was tagged to colloidal gold particles at pH 9.2. Postfixation with osmium tetroxide and embedding with Epon did not prevent the accessibility of the protein towards its corresponding binding sites. In plant nuclei, labeling was observed over the dense chromatin and to a lesser extent over the dispersed chromatin. Nucleolar labeling was preferentially located over the dense fibrillar component. Gold particles were also found to be associated with chloroplasts and mitochondria. In prokaryotic cells, a dispersed labeling was noted over the cytoplasm and, in some cases, the aggregation of few gold particles suggested the presence of packed DNA fibrils. Various control experiments confirmed the specificity of the labeling pattern obtained. Lactoferrin-gold complex appears to be a valuable probe for the intracellular demonstration of DNA molecules in double-fixed and Epon-embedded tissues.