The heterodimer acts as both an ATP-dependent DNA helicase and an ATP-dependent single-stranded exonuclease, acting in both directions. Recognizes the B.subtilis chi site (5'-AGCGG-3') which transforms the enzyme from a helicase which degrades both DNA strands to one with only 5' to 3' exonuclease activity. This generates a double-stranded DNA with a protruding 3'-terminated single-stranded tail suitable for the initiation of homologous recombination (chi fragment). The AddB nuclease domain is not required for chi fragment generation; this subunit has 5' -> 3' nuclease activity. RecA thread formation during DNA double-strand break repair requires RecJ or AddAB. ATP + H2O = ADP + H(+) + phosphate Binds 1 [4Fe-4S] cluster. At low magnesium concentrations there is no nuclease activity, but helicase activity is unaffected. Heterodimer of AddA and AddB. This enzyme is a functional homolog of the RecBCD enzyme; unlike the RecBCD enzyme it degrades both duplex strands symmetrically. Belongs to the helicase family. AddB/RexB type 1 subfamily.