In eukaryotes - in which replication, segregation and cell division are separated in time by checkpoint mechanisms, newly replicated sister chromatid loci remain associated until the initiation of chromosome segregation. The orderly association of sister chromatids is called sister chromatid cohesion and is mediated by proteins called cohesins. Cohesins are not found in bacteria, which display an integrated cell cycle in which segregation is concurrent with replication. Nevertheless, replicated sister chromatid loci have been shown to remain associated for a substantial length of time before their final separation. The length of time sister chromatid loci remain associated varies along the chromosome arms.
We have developed a method to monitor the orderly association of sister chromatids at a high-resolution and along the whole genome, Hi-SC2. In brief, interchromatid site-specific recombination serves as a measure of the frequency of contacts of sister chromatid loci. We use transposon mutagenesis to insert the detection system and deep sequencing to monitor the frequency of interchromatid recombination events.
We have applied Hi-SC2 to Vibrio cholerae, a model bacterium whose genome is divided between a typical bacterial chromosome and a domesticated mega-plasmid, chr1 and chr2, respectively. Sister chromatid cohesion was found to be elevated in the origin and terminus regions of both chromosomes. It was also elevated in a 100kbp region near the middle of one of the replication arms of chr1. We show that this is due to the presence of a pathogenicity island and to the binding of a nucleoid-associated protein, H-NS.