Figure 4.

Model of donor strand exchange and pilus elongation in vivo. An incoming chaperone:subunit complex binds to the amino-terminal domain (N) of the usher (step 1). The donor subunit (blue) then attacks (step 2) the acceptor subunit (grey). The transient complex in which the acceptor subunit is in contact with both the donor strand of the chaperone and the donor strand of the attacking subunit might represent a transition state or a short-lived reaction intermediate. After release of the chaperone from the acceptor subunit (step 3), the pilus translocates by the length of one subunit towards the cell surface (step 4).