RESUMO
CRISPR-Cas systems provide prokaryotes with acquired immunity against viruses and plasmids, but how these systems are regulated to prevent autoimmunity is poorly understood. Here, we show that in the S. pyogenes CRISPR-Cas system, a long-form transactivating CRISPR RNA (tracr-L) folds into a natural single guide that directs Cas9 to transcriptionally repress its own promoter (Pcas). Further, we demonstrate that Pcas serves as a critical regulatory node. De-repression causes a dramatic 3,000-fold increase in immunization rates against viruses; however, heightened immunity comes at the cost of increased autoimmune toxicity. Using bioinformatic analyses, we provide evidence that tracrRNA-mediated autoregulation is widespread in type II-A CRISPR-Cas systems. Collectively, we unveil a new paradigm for the intrinsic regulation of CRISPR-Cas systems by natural single guides, which may facilitate the frequent horizontal transfer of these systems into new hosts that have not yet evolved their own regulatory strategies.
Assuntos
Proteína 9 Associada à CRISPR/metabolismo , Sistemas CRISPR-Cas/genética , Expressão Gênica , Homeostase/genética , RNA Guia de Cinetoplastídeos/genética , Autoimunidade/genética , Sequência de Bases , Sequência Conservada , Regulação para Baixo/genética , Modelos Genéticos , Mutação/genética , Óperon/genética , Regiões Promotoras Genéticas/genética , Streptococcus pyogenes/genética , Estresse Fisiológico/genética , Transcrição Gênica , Ativação Transcricional/genéticaRESUMO
CRISPR-Cas immune systems provide bacteria with adaptive immunity against bacteriophages, but they are often transcriptionally repressed to mitigate auto-immunity. In some cases, CRISPR-Cas expression increases in response to a phage infection, but the mechanisms of induction are largely unknown, and it is unclear whether induction occurs strongly and quickly enough to benefit the bacterial host. In S. pyogenes, Cas9 is both an immune effector and auto-repressor of CRISPR-Cas expression. Here, we show that phage-encoded anti-CRISPR proteins relieve Cas9 auto-repression and trigger a rapid increase in CRISPR-Cas levels during a single phage infective cycle. As a result, fewer cells succumb to lysis, leading to a striking survival benefit after multiple rounds of infection. CRISPR-Cas induction also reduces lysogeny, thereby limiting a route for horizontal gene transfer. Altogether, we show that Cas9 is not only a CRISPR-Cas effector and repressor but also a phage sensor that can mount an anti-anti-CRISPR transcriptional response.