Figure 3.

Electron microscopic images of usher-based assembly intermediates. (A and B) Row 1, FimD alone; row 2, FimDCH complexes; row 3, FimDCGH complexes; row 4, FimDCFGH complexes; row 5 (and 5′ in B), FimD–pili (FimDCAFGH) complexes. Column Z contains models of the various assembly intermediates shown in column X, except the model in 5Z, which is a model of 5Y and 5′. (A) Negatively stained complexes. The images shown in row 1 illustrate two orientations of the usher. Face-on views (1Y) are common only in samples with FimD alone. The usher is positioned at the right-hand side of the images in rows 2–5 with fibers extending to the left. (B) Unidirectionally shadowed complexes. The ushers are positioned as in A except in 2X, 4X, and 4Y, in which the fibers are extending out toward the reader. The ushers in row 3 are at the bottom, with fibers extending upward. Most ushers adhered to the grid en face; however, an edge-on view is seen in 3Y. The arrow in 3Y indicates a portion of unraveled helical rod structure in the FimDCGH assembly intermediate. 5Y is an enlarged view of the usher seen in row 5′. The long arrow indicates the ring-shaped usher; the short arrow indicates the pilus rod; the arrowhead indicates a linear fiber emerging from the central pore of the usher. (Bar in A = 250 Å, applicable to all of the images except 5′; bar in 5′ = 500 Å.)