The discovery and characterization of the prokaryotic CRISPR-Cas immune system has led to a revolution in genome editing and engineering technologies. Despite the fact that most applications emerged after the discovery of the type II-A CRISPR-Cas9 system of Streptococcus pyogenes , its biological importance in this organism has received little attention. Here, we provide a comprehensive overview of the current knowledge about CRISPR-Cas systems from S. pyogenes . We discuss how the interplay between CRISPR-mediated immunity and horizontal gene transfer might have modeled the evolution of this pathogen. We review the current literature about the CRISPR-Cas systems present in S. pyogenes (types I-C and II-A), and describe their distinctive biochemical and functional features. Finally, we summarize the main biotechnological applications that have arisen from the discovery of the CRISPR-Cas9 system in S. pyogenes .

Polypeptide aggregation into amyloid is linked with several debilitating human diseases. Despite the inherent risk of aggregation-induced cytotoxicity, bacteria control the export of amyloid-prone subunits and assemble adhesive amyloid fibres during biofilm formation. An Escherichia protein, CsgC potently inhibits amyloid formation of curli amyloid proteins. Here we unlock its mechanism of action, and show that CsgC strongly inhibits primary nucleation via electrostatically-guided molecular encounters, which expands the conformational distribution of disordered curli subunits. This delays the formation of higher order intermediates and maintains amyloidogenic subunits in a secretion-competent form. New structural insight also reveal that CsgC is part of diverse family of bacterial amyloid inhibitors. Curli assembly is therefore not only arrested in the periplasm, but the preservation of conformational flexibility also enables efficient secretion to the cell surface. Understanding how bacteria safely handle amyloidogenic polypeptides contribute towards efforts to control aggregation in disease-causing amyloids and amyloid-based biotechnological applications.

Beyond defense against foreign DNA, the CRISPR-Cas9 system of pathogenic Francisella 23 novicida represses expression of an endogenous immunostimulatory lipoprotein and is essential for virulence. We investigated the specificity and molecular mechanism of this regulation, demonstrating that Cas9 has a highly specific regulon of four genes which must be repressed for bacterial virulence. Regulation occurs through a PAM-dependent interaction of Cas9 with its endogenous DNA targets, directed by a non-canonical small RNA (scaRNA) duplexed with tracrRNA. The limited complementarity between scaRNA and the endogenous DNA targets precludes cleavage. This highlights the evolution of the scaRNA to direct transcriptional interference via interaction with endogenous DNA without lethally targeting the chromosome.We show that scaRNA can be reprogrammed to repress other genes, and with engineered, extended complementarity to an exogenous target, the repurposed scaRNA:tracrRNA-Cas9 machinery can also be licensed to direct cleavage of target DNA. Natural Cas9 transcriptional interference likely represents a broad paradigm of regulatory functionality, which is potentially critical to the physiology of numerous Cas9-encoding pathogenic and commensal organisms.