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Abstract
Prokaryotes rely on varied mechanisms to protect themselves from viruses and other mobile genetic elements, including restriction modification, abortive infection, toxin/antitoxin, and CRISPR-Cas systems. The CRISPR-associated protein Csx1 contains a higher eukaryotes and prokaryotes nucleotide-binding (HEPN) domain and a highly conserved motif that is predicted to have ribonuclease activity. This motif is found in various ribonucleases associated with prokaryotic defense systems. Biochemical analysis of Csx1 revealed that it is a single strand-specific endoribonuclease that cleaves specifically after adenosines on the 3 side of the phosphodiester bond. Furthermore, mutation of the highly conserved residues of the predicted active site significantly decreased the observed cleavage activity of Csx1.