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

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