Involved in DNA double-strand break repair. Is not involved in recombination during natural competence or in plasmid establishment. The heterodimer acts as both an ATP-dependent DNA helicase and an ATP-dependent, dual-direction single-stranded exonuclease. Recognizes the chi site generating a DNA molecule suitable for the initiation of homologous recombination. This subunit has 5' -> 3' nuclease activity. ATP + H2O = ADP + H(+) + phosphate Heterodimer of AddA and RexB. Cells lacking this gene grow poorly, are more sensitive to UV light than wild-type cells and have reduced double-strand exonuclease activity. Belongs to the helicase family. AddB/RexB type 2 subfamily.