  A prominent organizational feature of bacterial chromosomes was revealed by
Hi-C experiments, indicating anomalously high contacts between the left and
right chromosomal arms. These long-range contacts have been attributed to
various nucleoid-associated proteins, including the ATPase SMC condensin.
Although the molecular structure of these ATPases has been mapped in detail, it
still remains unclear by which physical mechanisms they collectively generate
long-range chromosomal contacts. Here, we develop a computational model that
captures the subtle interplay between molecular-scale activity of slip-links
and large-scale chromosome organization. We first consider a scenario in which
the ATPase activity of slip-links regulates their DNA-recruitment near the
origin of replication, while the slip-link dynamics is assumed to be diffusive.
We find that such diffusive slip-links can collectively organize the entire
chromosome into a state with aligned arms, but not within physiological
constraints. However, slip-links that include motor activity are far more
effective at organizing the entire chromosome over all length-scales. The
persistence of motor slip-links at physiological densities can generate large,
nested loops and drive them into the bulk of the DNA. Finally, our model with
motor slip-links can quantitatively account for the rapid arm--arm alignment of
chromosomal arms observed in vivo.
