Type I-F CRISPR-Cas distribution and array dynamics in Legionella pneumophila

Abstract

9In bacteria and archaea, several distinct types of CRISPR-Cas systems provide adaptive 10 immunity through broadly similar mechanisms: short nucleic acid sequences derived from 11 foreign DNA, known as spacers, engage in complementary base pairing with invasive genetic 12 elements setting the stage for nucleases to degrade the target DNA. A hallmark of type I 13 CRISPR-Cas systems is their ability to acquire spacers in response to both new and previously 14 encountered invaders (naïve and primed acquisition, respectively). Our phylogenetic analyses of 15 47 L. pneumophila type I-F CRISPR-Cas systems and their resident genomes suggest that many 16 of these systems have been horizontally acquired. These systems are frequently encoded on 17 plasmids and can co-occur with nearly identical chromosomal loci. We show that two such co-18

CRISPR-Cas systems are widespread accessory elements across bacterial and archaeal plasmids

“…Their relatively higher occurrence on plasmids could indicate that CRISPR-Cas systems erode faster on plasmids, or that orphan components are recruited and/or selectively maintained to perform important, but as yet unknown, biological functions. Orphan CRISPR arrays could, for instance, employ host Cas machinery in trans (Almendros et al, 2016;Deecker and Ensminger, 2020) or facilitate plasmid chromosome integration via recombination between plasmid and host-encoded CRISPRs (Varble et al, 2019(Varble et al, , 2020. On the other hand, the higher proportion of orphan components may be an artefact of CRISPR-Cas prediction tools unable to detect a conceivably greater diversity of uncharted (sub)types across plasmids.…”