EM localization of an intron position close to the cleaved 5′ splice site in the C complex. Labeling was as described in Fig. 3, except that here the biotinylated label intron 5′ splice site oligonucleotide was used. (A) General field of C complexes incubated with PAG either in the absence (upper panel) or presence (lower panel) of the biotinylated oligonucleotide complementary to the intron close to the 5′ splice site. (B) Gallery of PAG-labeled C complexes seen in the dominant view. (C) Left: Superposition of all 111 gold-labeled C complexes used for locating the intron position close to the cleaved 5′ splice site, showing the dominant projection. All of the typical structural features of the dominant view are seen. Second from left: Outline sketch of the dominant view of the C complex with all gold positions indicated. Third from left: An example of the use of the 17-nm rule for three gold labels. Far right: Using the 17-nm rule for all 111 gold labels, the biotin-labeled nucleotide positioned on the intron close to its 5′ end localizes within the blue area close to domain 6 (labeled) of the C complex.
