Homologous recombination is an essential biological process that is involved in DNA repair and in the maintenance of genome integrity. The RecA protein is a key component of recombinational systems in bacteria. Its homologs are also present in higher organisms including humans. RecA functions in the form of nucleoprotein filaments that are assembled on single-stranded DNA, and which catalyze pairing and strand exchange between two homologous DNA duplexes. In Escherichia coli, the RecBCD and RecFOR protein complexes mediate RecA filament assembly, and consequently, play an important role in recombination. We have recently discovered that some E. coli mutants recombine quite efficiently in the absence of both RecBCD and RecFOR complexes. This recombination is RecA-dependent suggesting the existence of an alternative RecA loading activity. The aim of the project is to genetically characterize this RecBCD- RecFOR-independent (RecBFI) recombination pathway, and gain insight into this novel mechanism of RecA filament assembly. Although RecA is crucial for efficient recombination in bacteria, the recA mutants of radiation-resistant bacterium Deinococcus radiodurans display a significant residual ability to repair double-strand DNA breaks. Recent results from our group have shown that this RecA-independent repair is quite inaccurate leading to gross chromosome rearrangements. Our project is to identify the key genes/proteins involved in this RecA-independent repair, and to characterize, at the sequence level, gross genome rearrangements in D. radiodurans. Given that recombination is a fundamental process largely conserved during evolution, our research on bacteria may reveal molecular mechanisms that are applicable to eukaryotic recombination systems. Thus, the results of our project could be instructive for research on cancer and other human hereditary diseases related to defects in DNA recombination functions.