  Prokaryotes do not make use of a nucleus membrane to segregate their genetic
material from the cytoplasm, so that their nucleoid is potentially free to
explore the whole volume of the cell. Nonetheless, high resolution images of
bacteria with very compact nucleoids show that such spherical nucleoids are
invariably positioned at the center of mononucleoid cells. The present work
aims to determine whether such preferential localization results from generic
(entropic) interactions between the nucleoid and the cell membrane or instead
requires some specific mechanism, like the tethering of DNA at mid-cell or
periodic fluctuations of the concentration gradient of given chemical species.
To this end, we performed numerical simulations using a coarse-grained model
based on the assumption that the formation of the nucleoid results from a
segregative phase separation mechanism driven by the de-mixing of the DNA and
non-binding globular macromolecules. These simulations show that the abrupt
compaction of the DNA coil, which takes place at large crowder density, close
to the jamming threshold, is accompanied by the re-localization of the DNA coil
close to the regions of the bounding wall with the largest curvature, like the
hemispherical caps of rod-like cells, as if the DNA coil were suddenly
acquiring the localization properties of a solid sphere. This work therefore
supports the hypothesis that the localization of compact nucleoids at regular
cell positions involves either some anchoring of the DNA to the cell membrane
or some dynamical localization mechanism.
