| Summary: | The cell division of the alfalfa symbiont, <i>Sinorhizobium meliloti</i>, is dictated by a cell cycle regulatory pathway containing the key transcription factors CtrA, GcrA, and DnaA. In this study, we found that NtrX, one of the regulators of nitrogen metabolism, can directly regulate the expression of <i>ctrA</i>, <i>gcrA</i>, and <i>dnaA</i> from the cell cycle pathway. Three sets of <i>S. meliloti</i> <i>ntrX</i> mutants showed similar cell division defects, such as slow growth, abnormal morphology of some cells, and delayed DNA synthesis. Transcription of <i>ctrA</i> and <i>gcrA</i> was upregulated, whereas the transcription of <i>dnaA</i> and <i>ftsZ1</i> was downregulated in the insertion mutant and the strain of Sm1021 expressing <i>ntrX</i><sup>D53E</sup>. Correspondingly, the inducible transcription of <i>ntrX</i> activates the expression of <i>dnaA</i> and <i>ftsZ1</i>, but represses <i>ctrA</i> and <i>gcrA</i> in the depletion strain. The expression levels of CtrA and GcrA were confirmed by Western blotting. The transcription regulation of these genes requires phosphorylation of the conserved 53rd aspartate in the NtrX protein that binds directly to the promoter regions of <i>ctrA</i>, <i>gcrA</i>, <i>dnaA,</i> and <i>ftsZ1</i> by recognizing the characteristic sequence CAAN<sub>2-5</sub>TTG. Our findings suggest that NtrX affects <i>S. meliloti</i> cell division by regulating the transcription of the key cell cycle regulatory genes.
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