| Summary: | The CRISPR/Cas system protects bacteria against bacteriophage and plasmids through a sophisticated mechanism where <i>cas</i> operon plays a crucial role consisting of <i>cse1</i> and <i>cas3</i>. However, comprehensive studies on the regulation of <i>cas3</i> operon of the Type I-E CRISPR/Cas system are scarce. Herein, we investigated the regulation of <i>cas3</i> in <i>Escherichia coli</i>. The mutation in <i>gcvP</i> or <i>crp</i> reduced the CRISPR/Cas system interference ability and increased bacterial susceptibility to phage, when the <i>casA</i> operon of the CRISPR/Cas system was activated. The silence of the glycine cleavage system (GCS) encoded by <i>gcvTHP</i> operon reduced <i>cas3</i> expression. Adding <i>N<sup>5</sup></i>, <i>N<sup>10</sup></i>-methylene tetrahydrofolate (<i>N<sup>5</sup></i>, <i>N<sup>10</sup></i>-mTHF), which is the product of GCS-catalyzed glycine, was able to activate <i>cas3</i> expression. In addition, a cAMP receptor protein (CRP) encoded by <i>crp</i> activated <i>cas3</i> expression via binding to the <i>cas3</i> promoter in response to cAMP concentration. Since <i>N<sup>5</sup></i>, <i>N<sup>10</sup></i>-mTHF provides one-carbon unit for purine, we assumed GCS regulates <i>cas3</i> through associating with CRP. It was evident that the mutation of <i>gcvP</i> failed to further reduce the <i>cas3</i> expression with the <i>crp</i> deletion. These results illustrated a novel regulatory pathway which GCS and CRP co-regulate <i>cas3</i> of the CRISPR/Cas system and contribute to the defence against invasive genetic elements, where CRP is indispensable for GCS regulation of <i>cas3</i> expression.
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