The heterodimer acts as both an ATP-dependent DNA helicase and an ATP-dependent, dual-direction single-stranded exonuclease. Recognizes the L.lactis chi site (5'-GCGCGTG-3'), which stimulates homologous recombination. This subunit has 5'->3' exonuclease activity. ATP + H2O = ADP + H(+) + phosphate Binds 1 [4Fe-4S] cluster. Heterodimer of AddA and RexB. Belongs to the helicase family. AddB/RexB type 2 subfamily.