In bacteria and archaea, several distinct types of CRISPR-Cas systems provide adaptive immunity through broadly similar mechanisms: short nucleic acid sequences derived from foreign DNA, known as spacers, engage in complementary base pairing against invasive genetic elements setting the stage for nucleases to degrade the target DNA. A hallmark of type I CRISPR-Cas systems is their ability to acquire spacers in response to both new and previously encountered invaders (naive and primed

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... , respectively). In this work, we leverage the power of Legionella pneumophila, a genetically tractable, gram-negative bacterium and the causative agent of Legionnaires disease, to examine CRISPR array dynamics and the interplay between two extremely similar type I-F systems present in a single isolate. Using an established transformation efficiency assay, we show that the type I-F system in L. pneumophila is a highly protective system, with prominent spacer loss occurring in some transformed populations for both plasmid and chromosomal systems. Turning to next-generation sequencing, we demonstrate that, during a primed acquisition response, both systems acquire spacers in a strand-biased and directional manner, consistent with the patterns observed for previously studied type I-F systems in other bacterial species. We also show that the two systems can undergo cross-priming, whereby a target for one system can stimulate a primed acquisition response in the second. Finally, we combine these experimental data with bioinformatic analyses to propose a model in which cross-priming may replenish a depleted CRISPR array following a mass spacer deletion event.