A conserved mechanism for dimerization and activation of superfamily 1A UvrD-family helicases.

DNA helicases are ATP-dependent motor proteins that catalyze duplex DNA unwinding and are involved in DNA repair, recombination, and replication restart. Prominent members of the non-hexameric SF1A UvrD-family helicases are Escherichia coli UvrD, Rep, Bacillus stearothermophilus PcrA, and Mycobacterium tuberculosis (Mtb) UvrD1. SF1A monomers are processive 3'-to-5' single-stranded DNA translocases but need to be activated to become DNA helicases. One mechanism of activation is dimerization. Whereas Rep, UvrD, and PcrA form non-covalent dimers, the Mtb UvrD1 helicase forms a redox-dependent covalent dimer. Dimerization of Mtb UvrD1 occurs between the same regulatory domain (2B) within each subunit stabilized by a disulfide bond formed between the same cysteine (Cys451) within each subunit. Dimerization relieves an inhibitory interaction between the 2B domain and duplex DNA within the monomer-DNA complex. We show here that Rep, UvrD, and PcrA dimerize using the same 2B-2B interface. By placing a Cys residue within the 2B domains of Rep, UvrD, and PcrA in the structurally equivalent position occupied by Cys451 of Mtb UvrD1, all three enzymes form redox-dependent covalent dimers that are constitutively active helicases with increased processivity compared to the non-covalent dimers. Hence, the 2B domain is a general dimerization domain for UvrD-family SF1A helicases.