Comprehensive analysis of CRISPR array repeat mutations reveals subtype-specific patterns and links to spacer dynamics

Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) and their associated CRISPR-associated protein (Cas) systems are adaptive immune mechanisms in bacteria and archaea that protect against invading genetic elements by integrating short fragments of foreign DNA into CRISPR arrays. These arrays consist of repetitive sequences interspersed with unique spacers, guiding Cas proteins to recognize and degrade matching nucleic acids. The integrity of these repeat sequences is crucial for the proper function of CRISPR-Cas systems, yet their mutational dynamics remain poorly understood. In this study, we analyzed 56 343 CRISPR arrays across 25 628 diverse prokaryotic genomes to assess the mutation patterns in CRISPR array repeat sequences within and across different CRISPR subtypes. Our findings reveal, as expected to some extent, that mutation frequency is substantially higher in terminal repeat sequences compared to internal repeats consistently across system types. However, the mutation patterns exhibit an unexpected amount of variation among different CRISPR subtypes, suggesting that selective pressures and functional constraints shape repeat sequence evolution in distinct ways. Understanding these mutation dynamics provides insights into the stability and adaptability of CRISPR arrays across diverse bacterial and archaeal lineages. Additionally, we elucidate a novel relationship between repeat mutations and spacer dynamics, demonstrating that hotspots for terminal repeat mutations coincide with regions exhibiting higher spacer conservation. This observation corroborates recent findings indicating that spacer deletions occur at a frequency 374 times greater than that of mutations and are significantly influenced by repeat misalignment. Our findings suggest that repeat mutations might play a pivotal role in spacer retention or loss, or vice versa, thereby highlighting an evolutionary trade-off between the stability and adaptability of CRISPR arrays.