Accepted_test

CRISPR-cas systems are incredibly diverse and currently are classified into six major types and over 30 subtypes. Apart from their role in adaptive immunity, some of the CRISPR-cas subtypes are also involved in host gene regulation and collateral damage, leading to bacteriostatic or lethal outcomes for the host. CRISPR array spacers direct and influence canonical and non-canonical functions of the CRISPR-Cas system together with subtype Cas proteins. A better understanding of spacer adaptation mechanisms is crucial for uncovering the intricacies of an evolutionary arms race between prokaryotes and phages.

We present a large-scale analysis of CRISPR array spacers from 31845 complete bacterial genomes. Differences in melting energy and GC content between identified spacers, origin bacterial genomes, and infecting bacteriophages were explored. Spacers from the extremes of the GC content distribution were aligned to bacterial and infecting phage genomes to determine their origin. We further tested if spacer alignment locations to origin bacterial genomes correspond to predicted MGE and Prophage locations and if alignment scores between those spacer groups differ.

The GC content of the spacers was smaller than the GC content of the source bacterial genome but larger than the infecting viral genome. Alignments of the spacers from GC-rich distribution tails have shown their preferential targeting of host genomes. The alignment of GC-rich spacers to predicted prophage genomes confirms their viral origin and further supports the hypothesis of GC-rich spacers' preferential adaptation to CRISPR arrays.