Autoinhibition. (a) The RecA recombinase (see Chapter 13) forms filaments on DNA, characterized by distinct nucleation and filament extension phases. In the bacterial RecA protein, a C-terminal segment prevents efficient nucleation of binding to single-stranded DNA when single-stranded DNA–binding protein (SSB) is bound to the DNA strand. (b) If the C-terminal segment is removed (by recombinant DNA techniques; see Chapter 7) to create a truncated RecA protein, nucleation and subsequent displacement of SSB to form a filament on the DNA are rapid.