Prokaryotic antiviral defense mediated by CRISPR/Cas type I-E

Escherichia coli K12 contains a tightly regulated [1] type I CRISPR defense system that provides adaptive immunity against phage infection. This system functions by recognizing, processing and integrating short fragments of alien nucleic acid into repetitive CRISPR loci on the host genome [2]. These integrated sequences act as a memory, and protect their host against plasmids and viruses with cognate sequences. Results will be presented showing that virus-derived sequences contained in a CRISPR are used by CRISPR-associated (Cas) proteins from a bacterial host to mediate an antiviral response that counteracts infection [3]. After transcription of the CRISPR, a complex of Cas proteins (Cascade) specifically cleaves a CRISPR RNA precursor, and retains the cleavage products containing the virus-derived sequence. Recent progress in understanding Cascade mediated dsDNA recognition [4, 5] will be presented.