E. coli RecBC Helicase has Two Translocase Activities Controlled by a Single ATPase Motor

Escherichia coli RecBCD is a DNA helicase with two ATPase motors (RecB, a 3′ to 5′ translocase, and RecD, a 5′ to 3′ translocase) that functions in repair of double-stranded DNA breaks. The RecBC heterodimer, with only the RecB motor, remains a processive helicase. Here we examined RecBC translocation along single stranded (ss) DNA. Surprisingly, we find that RecBC displays two translocase activities: the primary translocase moves 3′ to 5′, while the secondary translocase moves RecBC along the opposite strand of a forked DNA at a similar rate. The secondary translocase is insensitive to the ssDNA backbone polarity, and we propose that its function may be to fuel RecBCD translocation along double stranded DNA ahead of the unwinding fork, and to ensure that the unwound single strands move through RecBCD at the same rate after interaction with a Chi sequence.