Figure 7.

Cross-priming between two similar CRISPR-Cas systems can repopulate a collapsed CRISPR array. A bacterium with a CRISPR-Cas system could undergo a mass spacer deletion event through homologous recombination between the first (black) and last (white) repeat sequences. The remaining locus carries a single repeat (white). While such “catastrophic collapse” events are likely to occur randomly at a certain rate, a driver of such a collapse could be the acquisition of a self-targeting spacer (yellow), selecting for spacer loss. Horizontal acquisition of a second CRISPR-Cas array (e.g., on a plasmid) is a first step toward replenishing the primary array. If cross-priming can occur between this secondary array and the collapsed array, the original CRISPR array is replenished, but bears an observable molecular scar - conversion of all the repeats to the sequence of the last repeat (white).