SMC progressively aligns chromosomal arms in Caulobacter crescentus but is antagonized by convergent transcription

The Structural Maintenance of Chromosomes (SMC) complex plays an important role in chromosome organization and segregation in most living organisms. In Caulobacter crescentus, SMC is required to align the left and the right arms of the chromosome that run in parallel down the long axis of the cell. However, the mechanism of SMC-mediated alignment of chromosomal arms remains elusive. Here, using genome-wide methods and microscopy of single cells, we show that Caulobacter SMC is recruited to the centromeric parS site and that SMC-mediated arm alignment depends on the chromosome partitioning protein ParB. We provide evidence that SMC likely tethers the parS-proximal regions of the chromosomal arms together, promoting arm alignment. Furthermore, we show that highly-transcribed genes near parS that are oriented against SMC translocation disrupt arm alignment, suggesting that head-on transcription interferes with SMC translocation. Our results demonstrate a tight interdependence of bacterial chromosome organization and global patterns of transcription. Overall design: Caulobacter crescentus were grown as described previously in Skerker et al., 2005 (PMID: 16176121). When appropriate, media were supplemented with antibiotics at the following concentrations (liquid/solid media) [ug/ml]): chloramphenicol (1/2), kanamycin (5/25), oxytetracycline (1/2), spectinomycin (25/100). Synchronizations were performed on mid-exponential phase cells using Percoll (GE Healthcare) and density gradient centrifugation as described previously in Jones et al., 2001 (PMID: 11283290). After synchronization, swarmer cells were fixed formadehyde for chromosome conformation capture and Hi-C analysis. For antibiotic treatments, swarmer cells were incubated with 25 ug/mL rifampicin (final concentration) for 30 minutes before fixing with formadehyde.