Crystal structure of the helicase domain from the replicative helicase-primase of bacteriophage T7

Helicases that unwind DNA at the replication fork are ring-shaped oligomeri c enzymes that move along one strand of a DNA duplex and catalyze the displ acement of the complementary strand in a reaction that is coupled to nucleo tide hydrolysis. The helicase domain of the replicative helicase-primase pr otein from bacteriophage T7 crystallized as a helical filament that resembl es the Escherichia coil RecA protein, an ATP-dependent DNA strand exchange factor. When viewed in projection along the helical axis of the crystals, s ix protomers of the T7 helicase domain resemble the hexameric rings seen in electron microscopic images of the intact T7 helicase-primase. Nucleotides bind at the interface between pairs of adjacent subunits where an arginine is near the gamma-phosphate of the nucleotide in trans. The bound nucleoti de stabilizes the folded conformation of a DNA-binding motif located near t he center of the ring. These and other observations suggest how conformatio nal changes are coupled to DNA unwinding activity.