Target discrimination and PAM profiling of the Thermotoga maritima type I-B CRISPR system

Here we reconstitute and characterize the type I-B1 system from the thermophile Thermotoga maritima (Tma) using purified components. We demonstrate that Tma Cascade requires standalone Cas11 expression, as the cryptic internal translation start site within cas8b1 is non-functional in E. coli.

The reconstituted system exhibits canonical type I function including RNA-guided DNA binding, PAM-dependent target discrimination, Cas3-mediated degradation, and seed region interrogation spanning seven PAM-proximal nucleotides. Using next-generation sequencing-based PAM library screens, we define a YYD consensus PAM (Y = C/T; D = G/A/T) with strong discrimination against the array repeat-adjacent sequence (AAC). Comprehensive PAM profiling reveals context-dependent tolerance for non-consensus sequences and identifies numerous intermediate-activity PAMs that may function in priming.