Fluorescence in situ hybridization (FISH) shows that DNA encoding ribosomal RNA cistron (rDNA) is localized to small speckles scattered in the nucleolus and the nucleolus-associated chromatin (NAC). This technique cannot however precisely locate rDNA in the nucleolar ultrastructural components such as the fibrillar center (FC), dense fibrillar component (DFC), and granular component (GC). In situ hybridization at the electron microscopic level is suitable for localization of rDNA at the ultrastructural level. We have tried to determine the precise localization of rDNA in the nucleolus of a higher plant by electron microscopic (EM) in situ hybridization using biotin-labeled 18S rDNA and demonstrated that it is exclusively localized in the fibrillar centers (FCs) and the nucleolus-associated chromatin (NAC). A secondary antibody coupled to the smallest (5nm) colloidal gold particles was used in this technique to increase the label. Another important factor to increase the label was pretreatment with proteinase. Convincing results are obtained when the samples are pretreated with 1μg/mL proteinase K for 45min at 37�C before immunogold labeling.
