FIG 5.

Dual function of type II CRISPR-Cas systems. (A) Genomic locus of type II CRISPR-Cas system. The cas operon consists of at least three genes (cas9, cas1, and cas2). A fourth gene (*) is present in type II-A (csn2) and II-B (cas4) systems but not in type II-C systems (122). Adjacent to the cas operon, the CRISPR locus is present (dark purple diamonds indicate repeats, and bright purple squares indicate the spacers), as well as the trans-encoded CRISPR RNA (tracrRNA) gene and possibly the recently proposed scaRNA gene (22). The order and orientation of the CRISPR and the genes vary in different genomes. (B) A role in defense against DNAs of invading genetic elements is well established (11), in which processed crRNA and a short version of the tracrRNA (most likely resulting from processing of a longer tracrRNA transcript or transcription from a second promoter; see panel A) eventually are responsible for interaction with target DNA. Eventually, both DNA strands are cleaved at the active sites of Cas9 (red triangles) (20, 129, 130). (C) A distinct role of Cas9 in virulence has been suggested (30), and a molecular basis for how Cas9 can codetermine virulence has been revealed (22): a long version of the tracrRNA shares significant homology with a target transcript, resulting in silencing and probably degradation of this transcript. Involvement of another small CRISPR-associated RNA (scaRNA) has been proposed; if indeed important, this scaRNA may be involved in stabilizing the interaction of the tracrRNA in the Cas9 complex.