Anoxia activates CRISPR-Cas immunity in the mouse intestine.

The natural context in which CRISPR-Cas systems are active in Enterobacteriaceae has remained enigmatic. Here we find that the Citrobacter rodentium type I-E CRISPR-Cas system is activated by the oxygen-responsive transcriptional regulator Fnr in the anoxic environment of the mouse intestine. Since Fnr-dependent regulation is predicted in ~41% of Enterobacteriaceae cas3 orthologues, we propose that anoxic regulation of CRISPR-Cas immunity is an adaptation that protects Enterobacteriaceae against threats from foreign DNA within the intestinal microbiome.