The RecBC enzyme loads recA protein onto ssDNA asymmetrically and independently of χ, resulting in constitutive recombination activation

Double-strand DNA break repair and homologous recombination in Escherichia coil proceed by the RecBCD pathway, which is regulated by cis- acting elements known as χ sites. A crucial feature of this regulation is the RecBCD enzyme-directed loading of RecA protein specifically onto the 3'- terminal, χ-containing DNA strand. Here we show that RecBC enzyme (lacking the RecD subunit) loads RecA protein constitutively onto the 3'-terminal DNA strand, with no requirement for χ. This strand is preferentially utilized in homologous pairing reactions. We propose that RecA protein loading is a latent property of the RecBCD holoenzyme, which is normally blocked by the RecD subunit and is revealed following interaction with χ.