A helicase/nuclease that prepares dsDNA breaks (DSB) for recombinational DNA repair. Binds to DSBs and unwinds DNA via a rapid (>1 kb/second) and highly processive (>30 kb) ATP-dependent bidirectional helicase. Unwinds dsDNA until it encounters a Chi (crossover hotspot instigator, 5'-GCTGGTGG-3') sequence from the 3' direction. Cuts ssDNA a few nucleotides 3' to Chi site, by nicking one strand or switching the strand degraded (depending on the reaction conditions). The properties and activities of the enzyme are changed at Chi. The Chi-altered holoenzyme produces a long 3'-ssDNA overhang which facilitates RecA-binding to the ssDNA for homologous DNA recombination and repair. In the holoenzyme this subunit contributes ssDNA-dependent ATPase and fast 5'-3' helicase activity. When added to pre-assembled RecBC greatly stimulates nuclease activity and augments holoenzyme processivity. Negatively regulates the RecA-loading ability of RecBCD. Exonucleolytic cleavage (in the presence of ATP) in either 5'- to 3'- or 3'- to 5'-direction to yield 5'-phosphooligonucleotides. In isolated subunit ATPase and 5'-3' helicase activity are inhibited by non-hydrolyzable ATP analogs and EDTA (PubMed:12815438). After reacting with DNA bearing a Chi site the holoenzyme is disassembled and loses exonuclease activity, DNA unwinding and Chi-directed DNA cleavage; RecB remains complexed with ssDNA, which may prevent holoenzyme reassembly (PubMed:10197988). High levels of Mg(2+) (13 mM MgCl(2+)) or incubation with DNase allow holoenyzme reassembly, suggesting it is DNA bound to RecB that prevents reassembly (PubMed:10197988). (Microbial infection) RecBCD is inhibited by the lambda virus gam protein (both GamL and GamS isoforms); in vitro a short preincubation prior to adding DNA results in maximal inhibition. Heterotrimer of RecB, RecC and RecD. All subunits contribute to DNA-binding. (Microbial infection) Lambda virus GamS protein interacts with the enzyme without displacing any of the subunits. The holoenzyme may undergo conformational shifts upon DNA binding: the nuclease domain of RecB may swing away from the DNA exit tunnel in RecC. When Chi DNA binds to the RecC tunnel the nuclease domain may then swing back to its original position (that seen in crystal structures), allowing it to nick the DNA 3' of the Chi site and then rotate to load RecA. At high Mg(2+) the nuclease domain may swing back more frequently, explaining differences seen in assays performed at high Mg(2+) (PubMed:25073102). As ssDNA is unwound and fed to the RecD subunit the latter transmits conformational changes through each subunit to activate the RecB nuclease (PubMed:27644322). Loss of RecBCD enzyme exonuclease activity, no effect on recombination proficiency or resistance to DNA-damaging agents (PubMed:10840065, PubMed:3526335). Cells can be transformed with retron Ec48-containing plasmids (PubMed:33157039). Belongs to the RecD family.