The DinG protein from Escherichia coli is a structure-specific helicase

The Escherichia coli DinG protein is a DNA damage-inducible member of the helicase superfamily 2. Using a panel of synthetic substrates, we have systematically investigated structural requirements for DNA unwinding by DinG. We have found that the helicase does not unwind blunt-ended DNAs or substrates with 3'-ss tails. On the other hand, the 5'-ss tails of 11-15 nucleotides are sufficient to initiate DNA duplex unwinding; bifurcated substrates further facilitate helicase activity. DinG is active on 5'-flap structures; however, it is unable to unwind 3'-flaps. Similarly to the homologous Saccharomyces cerevisiae Rad3 helicase, DinG unwinds DNA.RNA duplexes. DinG is active on synthetic D-loops and R-loops. The ability of the enzyme to unwind D-loops formed on superhelical plasmid DNA by the E. coli recombinase RecA suggests that D-loops may be natural substrates for DinG. Although the availability of 5'-ssDNA tails is a strict requirement for duplex unwinding by DinG, the unwinding of D-loops can be initiated on substrates without any ss tails. Since DinG is DNA damage-inducible and is active on D-loops and forked structures, which mimic intermediates of homologous recombination and replication, we conclude that this helicase may be involved in recombinational DNA repair and the resumption of replication after DNA damage.