Modulation of RecA nucleoprotein function by the mutagenic UmuD ' C protein complex

The RecA, UmuC, and UmuD' proteins are essential for error-prone, replicati ve bypass of DNA lesions. Normally, RecA protein mediates homologous pairin g of DNA. We show that purified Umu(D')(2)C blocks this recombination funct ion. Biosensor measurements establish that the mutagenic complex binds to t he RecA nucleoprotein filament with a stoichiometry of one Umu(D')(2)C comp lex for every two RecA monomers, Furthermore, Umu(D')(2)C competitively inh ibits 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 sever ely impedes DNA heteroduplex formation by RecA protein, ultimately blocking all DNA pairing activity and thereby abridging participation in recombinat ion function. Thus, Umu(D')(2)C restricts the activities of the RecA nucleo protein filament and presumably, in this manner, recruits it for mutagenic repair function. This modulation by Umu(D')(2)C is envisioned as a key even t in the transition from a normal mode of genomic maintenance by "error-fre e" recombinational repair, to one of "error-prone" DNA replication.