Chromosome search and rescue

Badrinarayanan et al. describe how regions of bacterial chromosomes come together to repair DNA breaks and then move back to their original positions once the process is complete.

Just like eukaryotes, bacteria repair DNA double-strand breaks (DSBs) by homologous recombination, which requires a broken chromosomal locus to pair up with the equivalent, undamaged region of its sister chromosome. To investigate how homologous loci find each other, and how they separate once DSBs are repaired, Badrinarayanan et al. induced single DSBs at defined locations along the 4-Mb chromosome of Caulobacter crescentus. This chromosome is tethered to one pole of the bacterium via a specific locus, called parS, located near the chromosome’s replication origin. During replication, one copy of parS is drawn to the opposite cell pole by the parS-binding protein ParB and the ATPase ParA.

When the researchers introduced a DSB near the replication origin of one of the chromosomes, ∼250 kb of the surrounding DNA left the pole and “searched” around the cell before finding the homologous, undamaged region at the opposite end of the cell. After ∼30 minutes, the homologous loci separated, and the repaired DNA was returned to its own pole by parS, ParB, and ParA. The rest of the chromosome retained its organization throughout this process.

When the researchers introduced DSBs further away from the replication origin, the damaged loci also sought out their intact copies. Their subsequent resegregation didn’t depend on the ParABS system, however. Senior author Michael Laub thinks that, instead, physical forces may cause these regions to snap back into position once homologous recombination is complete.