A physical approach to segregation and folding of the Caulobacter crescentus genome

Bacterial genomes are functionally organized. This organization is dynamic and globally changing throughout the cell cycle. Upon initiation of replication of the chromosome, the two origins segregate and move towards their new location taking along the newly replicated genome. Caulobacter crescentus employs a dedicated active partitioning (Par) system to move one copy of the parS centromere to the distal pole, while the other stays at the stalked pole. In this issue of Molecular Microbiology, Hong and McAdams describe studies on the speed of segregation of parS and regions up to 150 kb away. They show clear differences in segregation rates between parS and 50 kb flanking regions versus regions further away. To assess segregation rates the authors track fluorescent markers during movement using time-lapse microscopy. The relation between genomic and physical distance of pairs of markers reflects how the genome is folded. This relation permits testing experimental data against models from polymer physics. Such models are helpful in understanding principles of genome folding. Although long used in studies on eukaryotes, this approach has rarely been applied to bacteria. Finally, the authors give the first direct evidence for a role of the bacterial chromatin protein HU in folding the genome in vivo.