A double-edged sword? Quantifying tradeoffs in CRISPR immunity

CRISPR-Cas-based adaptive immune systems are ubiquitous in bacteria and archaea.

Paradoxically, CRISPR-associated endonucleases are remarkably slow at locating target DNA, with search times on the order of hours per molecule.

This implies that CRISPR systems must be very highly expressed in order to be effective, and indeed, expression of CRISPR-cas genes has been shown to measurably inhibit cell growth.

The proposed project will explore the nature of trade-offs in resource allocation between growth and defense using the Escherichia coli Type I-E CRISPR-cas system.

E. coli cells expressing fluorescently labeled Cascade will be grown in microfluidic chips, then challenged with a conjugative plasmid bearing an orthogonal fluorescent marker to facilitate detection of transconjugants.

This setup enables single-cell measurements of growth rate, Cascade concentration, and infection success probability, leading to detailed characterization of trade-offs between growth and defence as well as of the effect of cell-to-cell variability in Cascade expression on defence effectiveness.

Horizontal gene transfer via mobile genetic elements (MGEs) has broad ecological ramifications ranging from metabolism to antibiotic resistance, and the proposed work will shed light on the effect on CRISPR-Cas systems on the dynamics of MGE spread.

The project will seek to understand whether, amidst the panoply of bacterial defence systems, CRISPR-Cas is both exceptionally flexible and unusually costly.