Sinorhizobium meliloti is a soil bacterium that establishes a symbiosis within root nodules of legumes (Medicago sativa, for example) where it fixes atmospheric nitrogen into ammonia and obtains in return carbon sources and other nutrients. In this symbiosis, S. meliloti undergoes a drastic cellular change leading to a terminal differentiated form (called bacteroid) characterized by genome endoreduplication, increase of cell size and high membrane permeability. The bacterial cell cycle (mis)regulation is at the heart of this differentiation process. In free-living cells, the master regulator CtrA ensures the progression of cell cycle by activating cell division (controlled by the tubulin-like protein FtsZ) and simultaneously inhibiting supernumerary DNA replication, while on the other hand the downregulation of CtrA and FtsZ is essential for bacteroid differentiation during symbiosis, preventing endosymbiont division and permitting genome endoreduplication. Little is known in S. meliloti about regulators of CtrA and FtsZ, as well as the processes that control bacteroid development. Here, we combine cell biology, biochemistry and bacterial genetics approaches to understand the function(s) of FcrX, a new factor that controls both CtrA and FtsZ, in free-living growth and in symbiosis. Depletion of the essential gene fcrX led to abnormally high levels of FtsZ and CtrA and minicell formation. Using multiple complementary techniques, we showed that FcrX is able to interact physically with FtsZ and CtrA. Moreover, its transcription is controlled by CtrA itself and displays an oscillatory pattern in the cell cycle. We further showed that, despite a weak homology with FliJ-like proteins, only FcrX proteins from closely-related species are able to complement S. meliloti fcrX function. Finally, deregulation of FcrX showed abnormal symbiotic behaviors in plants suggesting a putative role of this factor during bacteroid differentiation. In conclusion, FcrX is the first known cell cycle regulator that acts directly on both, CtrA and FtsZ, thereby controlling cell cycle, division and symbiotic differentiation.

Sinorhizobium meliloti of the Alphaproteobacteria class has a fascinating spectrum of lifestyles, thriving as a free-living soil saprophyte, as an endophyte and as a nitrogen-fixing legume symbiont. In symbiosis, it undergoes a striking cellular differentiation process, which is controlled by the host plant through the activity of NCR peptides. NCRs interfere with the cell cycle of S. meliloti and transform the regular cycle consisting of strict successions of single DNA replication followed by cell division into an endoreduplication cycle of multiple genome duplications without divisions. This cellular differentiation results in giant and polyploid symbiotic bacterial cells that fix atmospheric nitrogen. Here we discuss the regulation of the free-living cell cycle in S. meliloti and present the hypothesis that the master regulator CtrA is the ultimate target of the NCR peptides, provoking the cell cycle switch in symbiosis.