RecA protein-dependent R-loop formation in vitro

The RecA protein of Escherichia coli, which has crucial roles in homologous recombination, DNA damage repair, induction of the SOS response, and SOS m utagenesis, was found to catalyze assimilation of complementary RNA into a homologous region of a DNA duplex (R-loop). The reaction strictly requires a region of mismatch in the duplex, which may serve as a nucleation site fo r RecA protein polymerization. The optimum conditions for the assimilation reaction resemble those for the previously studied RecA protein-catalyzed h omologous pairing and strand exchange reaction between two DNA molecules. O ur finding lends strong support to the proposal that RecA protein-catalyzed assimilation of a transcript into duplex DNA results in formation of an R- loop at certain regions of the chromosome and that, when stabilized, the R- loop can serve as an origin of chromosome replication.