Biology subjects

Keith, N. C.

Publications and source records attributed to Keith, N. C..

2 recordsLinked to original sources

bioRxiv2024

A dynamic subpopulation of CRISPR-Cas overexpressers allows Streptococcus pyogenes to rapidly respond to phage

Many CRISPR-Cas systems, which provide bacteria with adaptive immunity against phages, are transcriptionally repressed in their native hosts. How CRISPR-Cas expression is induced as needed, for example during a bacteriophage infection, remains poorly understood. In Streptococcus pyogenes, a non-canonical guide RNA tracr-L directs Cas9 to autorepress its own promoter. Here, we describe a dynamic subpopulation of cells harboring single mutations that disrupt Cas9 binding and cause CRISPR-Cas overexpression. Cas9 actively expands this population by elevating mutation rates at the tracr-L target site. Overexpressers exhibit higher rates of memory formation, stronger potency of old memories, and a larger memory storage capacity relative to wild-type cells, which are surprisingly vulnerable to phage infection. However, in the absence of phage, CRISPR-Cas overexpression reduces fitness. We propose that CRISPR-Cas overexpressers are critical players in phage defense, enabling bacterial populations to mount rapid transcriptional responses to phage without requiring transient changes in any one cell.

microbiology↗

bioRxiv2023

Anti-CRISPR proteins trigger a burst of CRISPR-Cas9 expression that enhances phage defense

CRISPR-Cas immune systems provide bacteria with adaptive immunity against bacteriophages, but they are often transcriptionally downregulated to mitigate autoimmunity. 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 autorepressor of CRISPR-Cas expression. Here, we show that phage-encoded anti-CRISPR proteins relieve Cas9 autorepression 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.

microbiology↗