DNA double-strand break (DSB) is a lethal lesion if left unrepaired, whereas its imprecise repair yields mutations and genomic rearrangements, which in humans may cause cancer and premature ageing. DSB repair is an evolutionary conserved process and hence is studied using model organisms, such as Escherichia coli. In E. coli, a process named homologous recombination (HR), wherein an intact homologous sequence serves as a template, repairs a DSB. HR is effected by a 3’-ovehang, which is produced by degrading its complementary 5’-ending strand in a process called DSB resection. Resection in E. coli is catalyzed by a RecBCD helicase/nuclease, which unwinds DNA and degrades both the unwound strands. Upon its interaction with Chi sequence of the processed DNA, RecBCD ceases degradation of the 3’ strand and starts loading RecA recombinase onto it. This project deals with some yet uncharacterized aspects of DSB repair in E. coli, using genetic, molecular biology and bioinformatics methods. We plan to determine production and the role of SSB, an essential protein, in DSB repair, specifically in illegitimate recombination. In addition, we will continue our studies of the role of exonucleases in E. coli DSB repair, as well as their phylogenetic relations and distribution in bacteria. Moreover, we plan to explore whether DSB processing in E. coli is achieved by a single or multiple rounds of helicase loading onto DNA ends, and additionally, what is the role of RecA protein in RecBCD interaction with Chi sequence. The project envisages a fundamental research that due to conserved mechanisms of DSB repair may have a medical relevance in a sense of deepening our insight into origins of cancer and the means to treat it. Another medical aspect of the project stems from better characterization of bacterial defense mechanisms involved in bacterial interaction with their human host, having repercussion on their pathogenicity and resistance to antibiotics.