Genome plasticity is governed by double strand break DNA repair in Streptomyces

The linear chromosome of the bacterium Streptomyces exhibits a remarkable genetic organization with grossly a central conserved region flanked by variable chromosomal arms. The terminal diversity co-locates with an intense DNA plasticity including the occurrence of large deletions associated to circularization and chromosomal arm exchange. These observations prompted us to assess the role of double strand break (DSB) repair in chromosome plasticity following. For that purpose, DSBs were induced along the chromosome using the meganuclease I-SceI. DSB repair in the central region of the chromosome was mutagenic at the healing site but kept intact the whole genome structure. In contrast, DSB repair in the chromosomal arms was mostly associated to the loss of the targeted chromosomal arm and extensive deletions beyond the cleavage sites. While homologous recombination occurring between copies of DNA sequences accounted for the most part of the chromosome rescue events, Non Homologous End Joining was involved in mutagenic repair as well as in huge genome rearrangements (i.e. circularization). Further, NHEJ repair was concomitant with the integration of genetic material at the healing site. We postulate that DSB repair drives genome plasticity and evolution in Streptomyces and that NHEJ may foster horizontal transfer in the environment.