Electron microscopy of 37S exon and 45S B-like complexes. Overviews of negatively stained 37S exon (A) and 45S B-like (B) complexes or (C) affinity-purified 37S exon complexes incubated solely with the 5′ss oligo are shown on the left. Representative class averages of each complex are shown in the galleries on the right from most (1) to least (5) frequently observed classes. The relative abundance of the different classes of the 37S exon complex is indicated. Characteristic structural features of the body domain of the purified 37S exon complex + 5′ss oligo are indicated with arrows. (D) Typical class averages of an intron-defined B complex formed on the MINX pre-mRNA. Scale bars correspond to 50 nm. (E) Schematic representation of the 37S exon complex including different orientations of the head domain characteristic for galleries 1–2 (solid line), 3–4 (dashed), and 5 (dotted), and of the intron-defined B complex with main structural features labeled according to Boehringer et al. (2004).
