FIG. 3.

The RecBCD enzyme is a sequence-regulated DNA helicase-nuclease. Shown are details of the RecBCD-catalyzed DNA end-processing reaction. (1) RecBCD (light blue) binds tightly to a blunt (or nearly blunt) DNA end of a linear DNA duplex. (2) RecBCD couples the hydrolysis of ATP to DNA translocation and unwinding (i.e., helicase activity). The ssDNA products are cleaved asymmetrically, with the degradation of the 3′-terminated ssDNA tail being much more vigorous than the degradation of the complementary tail. (3) The enzyme continues to translocate until it pauses at a correctly oriented Chi sequence. The recognition of the Chi sequence dramatically alters the biochemical properties of the enzyme (indicated by color change to pink). (4) The enzyme continues to translocate, but the nuclease polarity is switched; the degradation of the 3′ ssDNA tail is attenuated, whereas the hydrolysis of the 5′ ssDNA tail is upregulated. After Chi recognition, RecBCD facilitates the loading of the RecA protein onto the 3′ ssDNA tail. (5) RecBCD repeatedly deposits RecA protomers, which act as nucleation points for filament growth primarily in the 5′→3′ direction. (6) The RecBCD enzyme dissociates from the DNA. The product of the enzyme is a recombinogenic nucleoprotein complex of the RecA protein bound to the 3′ ssDNA tail with Chi at its terminus. This product is able to invade homologous duplex DNA to promote the recombinational repair of a DSB or to restart DNA replication as appropriate. Note that this cartoon is not to scale; several thousand base pairs of DNA may be processed before and after the Chi sequence, and the RecA filament consists of many more protomers than are shown.