Fig. 6. Immunoelectron microscopic localisation of p40/50 along a transcriptionally active BR gene. Polytene chromosomes were isolated, incubated with a p40/50 antibody and detected with a secondary antibody conjugated to 12 nm colloidal gold markers. (A) Section through a Balbiani ring showing several segments of active genes. Examples of proximal (p), middle (m) and distal (d) segments have been indicated. Arrows mark the position of gold particles. (B) Schematic drawing of an active BR gene. Growing particles in the promoter-proximal (p), middle (m) and promoter-distal (d) portions of the gene are shown, as well as the chromatin axis. (C-G) Immunolabelled proximal (C), middle (D,E) and distal (F,G). segments of BR genes. Schematic drawings are shown below the electron micrographs, with examples of labelled nascent RNPs (dark) and unlabelled RNPs (outlined). The chromatin axis is indicated by a short dashed line. Bars, 100 nm.