Changes in nucleoid shape and structure as a function of the cell cycle

Deinococcus radiodurans is well known for its outstanding radiation resistance – it can withstand very high doses of ionizing radiation and many other DNA damaging agents. In this study, we discovered that D. radiodurans is also particularly well suited for live cell imaging. It is relatively large and a complete cell cycle lasts approximately two hours and is thus neither too slow nor too fast to capture the changes occurring at the cell and nucleoid level during growth and division of the bacteria.

With this study, we set out to evaluate the shape and dynamics of its nucleoid as a function of its cell cycle. We rapidly realized that D. radiodurans nucleoids do not only adopt a toroidal shape, but instead follow a well-defined choreography in which the ring-like structures are just one act of this ballet. Starting from a highly compact conformation just after cytokinesis, the nucleoids then expand to adopt a toroidal shape which progressively opens up into a crescent as the next round of cell division is initiated. As septal growth progresses the crescent-shaped nucleoids stretch along the longest axis of the cell and align orthogonally to the future division axis. At this stage the replicated genomes are pulled into the two daughter cells before septal closure and the newly formed nucleoids are ready to start a new cycle again. Much is known regarding chromosome condensation and segregation in eukaryotes, but so far these processes had never been visualized in bacteria. In this study, we clearly evidenced that bacterial nucleoids are not simple, static structures, but are instead highly complex structures that are tightly coupled to cell cycle progression.

Floc’h K, Lacroix F, Servant P, Wong YS, Kleman JP, Bourgeois D and Timmins J. Cell morphology and nucleoid dynamics in dividing D. radiodurans. Nat Commun. (2019) 10 (1). p.3815. DOI : 10.1038/s41467-019-11725-5