Three-dimensional organization of nuclear DNA in the higher plant nucleolonema studied by immunoelectron microscopy

Abstract

Serial ultrathin sectioning of Lowicryl-embedded Vicia faba cells followed by silver impregnation added some new evidence supporting a previous suggestion that a 350 nm-thick nucleolonema filament coils up and forms a 1 μm-thick solenoid. The axial region of the solenoid was seen as low electron-opaque patches by the electron microscope. These patches usually differentiated into two regions; a moderately electron-opaque outer region (fibrillar centre) and a transparent inner region (central space). Intranucleolar localization of DNA was studied by immunoelectron microscopy using a monoclonal anti-DNA antibody. The sensitive labelling was obtained when Tween 20 (a detergent) was used as a rinse and 5 nm gold particles were used as markers. In the nucleolus gold particles were most heavily concentrated in the fibrillar centres while the central space was completely devoid of them. In the dense fibrillar component, the label gradually increased in density towards the fibrillar centres. The results presented here suggest that the fibrillar centres are laterally associated with the nucleolonema filament and reside in the axial side of the nucleolonema solenoid.