RESEARCH ARTICLE Cell Cycle Control by the Master Regulator CtrA in Sinorhizobium meliloti

In all domains of life, proper regulation of the cell cycle is critical to coordinate genome rep-lication, segregation and cell division. In some groups of bacteria, e.g. Alphaproteobac-teria, tight regulation of the cell cycle is also necessary for the morphological and functional differentiation of cells. Sinorhizobium meliloti is an alphaproteobacterium that forms an economically and ecologically important nitrogen-fixing symbiosis with specific legume hosts. During this symbiosis S.meliloti undergoes an elaborate cellular differenti-ation within host root cells. The differentiation of S.meliloti results in massive amplification of the genome, cell branching and/or elongation, and loss of reproductive capacity. In Caulobacter crescentus, cellular differentiation is tightly linked to the cell cycle via the ac-tivity of the master regulator CtrA, and recent research in S.meliloti suggests that CtrA might also be key to cellular differentiation during symbiosis. However, the regulatory cir-cuit driving cell cycle progression in S.meliloti is not well characterized in both the free-living and symbiotic state. Here, we investigated the regulation and function of CtrA in S. meliloti. We demonstrated that depletion of CtrA cause cell elongation, branching and ge