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Abstract
Bacteriophages (or phages) are the most abundant biological entities on earth, and are estimated to outnumber their bacterial prey by tenfold^sub 1^. The constant threat of phage predation has led to the evolution of a broad range of bacterial immunity mechanisms that in turn result in the evolution of diverse phage immune evasion strategies, leading to a dynamic co-evolutionary arms race^sub 2,3^. Although bacterial innate immune mechanisms against phage abound, the only documented bacterial adaptive immune system is the CRISPR/Cas (clustered regularly interspaced short palindromic repeats/CRISPR-associated proteins) system, which provides sequence-specific protection from invading nucleic acids, including phage^sup 4-11^. Here we show a remarkable turn of events, in which a phage-encoded CRISPR/Cas system is used to counteract a phage inhibitory chromosomal island of the bacterial host. A successful lytic infection by the phage is dependent on sequence identity between CRISPR spacers and the target chromosomal island. In the absence of such targeting, the phage-encoded CRISPR/Cas system can acquire new spacers to evolve rapidly and ensure effective targeting of the chromosomal island to restore phage replication. [PUBLICATION ABSTRACT]