AddAB

AddAB is a bacterial DNA repair complex that initiates processing of double-strand breaks to generate a 3' single-stranded DNA substrate needed for homologous recombination. It belongs to the RecBCD family of DNA-processing enzymes and serves as an alternative to RecBCD in bacteria that rely on different repair pathways. The complex is a heterodimer composed of two subunits, AddA and AddB. AddA provides a DNA helicase motor and contributes nuclease activity, while AddB participates in DNA processing and helps engage the DNA end for resection. Together, AddAB resects the broken DNA end to produce a 3' overhang that is bound by single-strand DNA-binding proteins and handed off to the RecA recombinase for strand invasion and repair.

Regulation and specificity of AddAB can be influenced by sequence motifs known as Chi sites in some organisms, which modulate nuclease activity and bias processing toward regions of homology. The precise Chi motif and regulatory details vary across species, reflecting diversity in bacterial DNA repair strategies.

Biological role and distribution: AddAB is common in many Gram-positive bacteria, including Bacillus subtilis, and is found in other bacteria that utilize RecBCD-like pathways for double-strand-break repair. It contributes to genome maintenance, genetic recombination, and defense against phage infection by enabling efficient processing of DNA ends after damage. Disruption of AddAB function often reduces survival following DNA-damaging stress and lowers recombination efficiency.

History and context: The AddAB system was identified through studies of bacterial DNA repair and recognized as a functional analog of RecBCD, illustrating the diversity of double-strand-break processing mechanisms across bacteria. See also RecBCD and homologous recombination.