How E.coli organizes and segregates its chromosome
Seminar Room 1, Newton Institute
Live cell imaging of the E.coli nucleoid, illuminated with HupA-mCherry, reveals a well-defined helical ellipsoid that is trapped within the cell radially but not longitudinally. Basic elliposidal shape results from longitudinal density bundling while helicity results from interactions between these bundles and the cell periphery. Unexpectedly, the nucleoid exhibits two distinct types of cyclic dynamic changes, both independent of DNA replication: (1) On time scales of seconds-to-minutes, longitudinal density waves flux through the shape over distances comparable to the length of the nucleoid, resulting in dynamic shape changes. (2) At intervals of ~20min, the nucleoid exhibits ~10min pulses of chromosome elongation. These pulses, which are implemented by elongation-biased density waves, are temporally and functionally linked to step-wise separation of sister chromosomes. The presented findings support a two-component model for sister segregation and the existence of nucleoid stress cycles which, we propose, function to release the nucleoid from linkages that constrain both morphogenetic evolution and separation of sisters. These cycles could comprise a primordial cell cycle, and the same principles could pertain broadly across evolutionary space and time.