The active form of DNA polymerase V is UmuD′2C–RecA–ATP

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Abstract
DNA-damage-induced SOS mutations arise when Escherichia coli DNA polymerase (pol) V, activated by a RecA nucleoprotein filament (RecA*), catalyses translesion DNA synthesis. Here we address two longstanding enigmatic aspects of SOS mutagenesis, the molecular composition of mutagenically active pol V and the role of RecA*. We show that RecA* transfers a single RecA–ATP stoichiometrically from its DNA 3′-end to free pol V (UmuD′2C) to form an active mutasome (pol V Mut) with the composition UmuD′2C–RecA–ATP. Pol V Mut catalyses TLS in the absence of RecA* and deactivates rapidly upon dissociation from DNA. Deactivation occurs more slowly in the absence of DNA synthesis, while retaining RecA–ATP in the complex. Reactivation of pol V Mut is triggered by replacement of RecA–ATP from RecA*. Thus, the principal role of RecA* in SOS mutagenesis is to transfer RecA–ATP to pol V, and thus generate active mutasomal complex for translesion synthesis.