Abstract
CRISPR-Cas systems provide acquired immunity in prokaryotes. Upon infection, short sequences from the phage genome, known as spacers, are inserted between the CRISPR repeats. Spacers are transcribed into small RNA molecules that guide nucleases to their targets. The forces that shape the distribution of newly acquired spacers, which is observed to be uneven, are poorly understood. We studied the spacer patterns that arise after phage infection of Staphylococcus aureus harboring the Streptococcus pyogenes type II-A CRISPR-Cas system. We observed that spacer patterns are established early during the CRISPR-Cas immune response and correlate with spacer acquisition rates, but not with spacer targeting efficiency. The rate of spacer acquisition depended on sequence elements within the spacer, which in turn determined the abundance of different spacers within the adapted population. Our results reveal how the two main forces of the CRISPR-Cas immune response, acquisition and targeting, affect the generation of immunological diversity.