FIG 2.

Overview of the three type II CRISPR-Cas subtypes. All three subtypes share a conserved set of cas genes: cas1, cas2, and cas9. Type II-A has an additional csn2 gene, and type II-B has an additional cas4 gene (21). Type II-C does not feature an additional cas gene beyond the three conserved cas genes (122). All subtypes feature a small trans-encoded RNA called trans-activating CRISPR RNA (tracrRNA); type II-C displays variation in the location of tracrRNA. cas9, cas1, and cas2 are indicated with green arrows, and tracrRNA is shown with yellow boxes. Transcription start sites are shown as black arrows upstream of the repeats (red diamonds) and spacers (purple squares) in type II-A (e.g., in Streptococcus spp.) and -B (e.g., in Legionella pneumophila) CRISPR loci, or within each spacer in the case of the minimal type II-C CRISPR systems of Neisseria meningitidis (upper) and Campylobacter jejuni (lower) (122).