A conundrum resolved: regulation and activation of UvrD-family DNA helicases/translocases

Superfamily 1 helicases are conserved nonhexameric ATP-dependent enzymes that unwind DNA and RNA duplexes processively or remove proteins, playing critical roles in DNA repair, replication, recombination, and RNA processing. While crystal structures of Superfamily 1A UvrD-family helicases suggested that monomers are active helicases requiring an essential 2B regulatory domain-DNA interaction, biochemical studies show that helicase activation requires dimerization. Recent cryo-electron microscopy (EM) structures of Mycobacterium tuberculosis UvrD1 dimers reveal that dimerization involves the 2B domains, eliminating their inhibitory interaction with duplex DNA, contradicting these original models. Escherichia coli UvrD dimers use the same dimerization interface, suggesting a general mechanism for this class of helicases. Herein, we describe how these results require re-evaluation of helicase mechanisms that were based on the monomeric structures alone.