Interrogating two extensively self-targeting Type I CRISPR-Cas systems in Xanthomonas albilineans reveals distinct anti-CRISPR proteins that block DNA degradation
CRISPR-Cas systems store fragments of invader DNA as spacers to recognize and clear those same invaders in the future. Spacers can also be acquired from the host's genomic DNA, leading to lethal self-targeting. While self-targeting can be circumvented through different mechanisms, natural examples remain poorly explored. Here, we investigate extensive self-targeting by two CRISPR-Cas systems encoding an astonishing 24 self-targeting spacers in the plant pathogen Xanthomonas albilineans. We show, based on transcriptomics analyses, that the native I-C and I-F1 systems are actively expressed and that CRISPR RNAs are properly processed. When expressed in Escherichia coli, each Cascade complex binds its PAM-flanked DNA target to block transcription, while the addition of Cas3 paired with genome targeting induces cell killing. While exploring how X. albilineans survives self-targeting, we predicted putative anti-CRISPR proteins (Acrs) encoded within the bacterium's genome. Screening of identified candidates with cell-free transcription-translation systems and in E. coli revealed two Acrs, which we named AcrIC11 and AcrIF12Xal, that inhibit Cas3 but not Cascade of the respective system. These findings reveal how a bacterium tolerates extensive self-targeting through two CRISPR-Cas systems and expand the suite of Cas3-inhibiting Acrs. Overall design: To assess CRISPR array processing and cas gene expression in X. albilineans, we performed RNA-Seq and small RNA-Seq in duplicates.