Direct regulation of cell cycle regulatory gene expression by NtrX to promoteSinorhizobium meliloticell division

Abstract

In α-proteobacteria, the CtrA signaling pathway regulates cell cycle progression. A species whose cell duplication is associated with CtrA stability is affected by the response regulator NtrX. However, the function of NtrX acting on the cell cycle regulation in bacteria remains unclear. Here, we report that NtrX controls transcription of the CtrA system genes involved in cell cycle regulation in a legume symbiont, Sinorhizobium meliloti. Three groups of ntrX mutants showed the similar cell cycle defects, such as slow growth, abnormal shapes, and irregular genomic DNA accumulation. Expression of the CtrA signaling pathway genes including ctrA, gcrA, dnaA, divL and cpdR1, is differentially regulated by the phosphorylated NtrX protein. The regulation is achieved through direct protein-DNA interactions. The 53rd aspartate residue known as the conserved phosporylation site and located in the receiver domain of NtrX, is required for S. meliloti cell cycle regulation. Interestingly, expression of S. meliloti ntrX derivatives in Caulobacter and Agrobacterium strains showed distinct defects of cell duplication and growth, suggesting that NtrX plays different roles in cell cycle regulation in these bacteria. Our findings demonstrate that NtrX is an upstream transcriptional regulator of the CtrA signaling pathway in S. meliloti, which could be associated with nitrogen nutrient response.