Modulation of RecA Nucleoprotein Function by the Mutagenic UmuD′C Protein Complex *

The RecA, UmuC, and UmuD′ proteins are essential for error-prone, replicative bypass of DNA lesions. Normally, RecA protein mediates homologous pairing of DNA. We show that purified Umu(D′) 2 C blocks this recombination function. Biosensor measurements establish that the mutagenic complex binds to the RecA nucleoprotein filament with a stoichiometry of one Umu(D′) 2 C complex for every two RecA monomers. Furthermore, Umu(D′) 2 C competitively inhibits LexA repressor cleavage but not ATPase activity, implying that Umu(D′) 2 C binds in or proximal to the helical groove of the RecA nucleoprotein filament. This binding reduces joint molecule formation and even more severely impedes DNA heteroduplex formation by RecA protein, ultimately blocking all DNA pairing activity and thereby abridging participation in recombination function. Thus, Umu(D′) 2 C restricts the activities of the RecA nucleoprotein filament and presumably, in this manner, recruits it for mutagenic repair function. This modulation by Umu(D′) 2 C is envisioned as a key event in the transition from a normal mode of genomic maintenance by “error-free” recombinational repair, to one of “error-prone” DNA replication.