Clustered Regularly Interspaced Short Palindromic Repeat (CRISPR) RNAs in the Porphyromonas gingivalis CRISPR-Cas I-C System.

The CRISPR-Cas (clustered regularly interspaced short palindromic repeat-CRISPR-associated protein) system is unique to prokaryotes and provides the majority of bacteria and archaea with immunity against nucleic acids of foreign origin. CRISPR RNAs (crRNAs) are the key element of this system, since they are responsible for its selectivity and effectiveness. Typical crRNAs consist of a spacer sequence flanked with 5' and 3' handles originating from repeat sequences that are important for recognition of these small RNAs by the Cas machinery. In this investigation, we studied the type I-C CRISPR-Cas system in Porphyromonas gingivalis, a human pathogen associated with periodontitis, rheumatoid arthritis, cardiovascular disease, and aspiration pneumonia. We demonstrated the importance of the 5' handle for crRNA recognition by the effector complex and consequently activity, as well as secondary trimming of the 3' handle, which was not affected by modifications of the repeat sequence.IMPORTANCEPorphyromonas gingivalis, a clinically relevant Gram-negative, anaerobic bacterium, is one of the major etiologic agents of periodontitis and has been linked with the development of other clinical conditions, including rheumatoid arthritis, cardiovascular disease, and aspiration pneumonia. The presented results on the biogenesis and functions of crRNAs expand our understanding of CRISPR-Cas cellular defenses in P. gingivalis and of horizontal gene transfer in bacteria.