AbstractThe bacterial archetypal adaptive immune system, CRISPR-Cas, is thought to be repressed in the best-studied bacterium,Escherichia coliK-12. We show here that theE. coliCRISPR-Cas system is active and serves to inhibit its nine defective (i.e., cryptic) prophages. Specifically, compared to the wild-type strain, reducing the amounts of specific interfering RNAs (crRNA) decreases growth by 40%, increases cell death by 700%, and prevents persister cell resuscitation. Similar results were obtained by inactivating CRISPR-Cas by deleting the entire 13 spacer region (CRISPR array); hence, CRISPR-Cas serves to inhibit the remaining deleterious effects of these cryptic prophages, most likely through CRISPR array-derived crRNA binding to cryptic prophage mRNA, rather than through cleavage of cryptic prophage DNA; i.e., self-targeting. Consistently, four of the 13E. colispacers contain complementary regions to the mRNA sequences of seven cryptic prophages, and inactivation of CRISPR-Cas increases the level of mRNA for lysis protein YdfD of cryptic prophage Qin and lysis protein RzoD of cryptic prophage DLP-12. Also, lysis is clearly seen via transmission electron microscopy when the whole CRISPR-Cas array is deleted, and eliminating spacer #12, which encodes crRNA with complementary regions for DLP-12 (includingrzoD), Rac, Qin (includingydfD), and CP4-57 cryptic prophages, also results in growth inhibition and cell lysis. Therefore, we report the novel results that (i) CRISPR-Cas is active inE. coliand (ii) CRISPR-Cas is used to tame cryptic prophages, likely through RNAi; i.e., unlike with active lysogens, active CRISPR-Cas and cryptic prophages may stably co-exist.
