CRISPR-Cas Immunity against Phages: Its Effects on the Evolution and Survival of Bacterial Pathogens
CRISPR loci contain clusters of repeats (white boxes) and spacers (colored boxes) that are flanked CRISPR-associated (cas) genes. (A) During adaptation new spacers derived from the genome of the invading virus are incorporated into the CRISPR array by an unknown mechanism. Repeat duplication is also required. (B) During crRNA biogenesis a CRISPR precursor transcript is processed by Cas endoribonucleases within repeat sequences to generate small crRNAs. (C) During targeting the match between the crRNA spacer and target sequences specifies the nucleolytic cleavage of invading mobile genetic elements such as viruses and plasmids. (D) In the CRISPR-Cas system of F. novicida, the tracrRNA (a small RNA mediated in crRNA biogenesis in this system) contains homology to the BLP (bacterial lipoprotein) transcript. The base-pair interaction between the tracrRNA and the BLP mRNA (mediated also by another small RNA, the scaRNA, and the nuclease Cas9) regulates the expression of this immunomodulatory lipoprotein.