CRISPR Arrays Away from cas Genes

Area of Science:

• Genomics
• Microbiology
• Molecular Biology

Background:

• CRISPR-Cas systems are crucial for microbial immunity, typically comprising CRISPR arrays and cas genes.
• A significant portion of identified CRISPR arrays lack adjacent cas genes, posing identification challenges due to false positives from repetitive sequences.

Purpose of the Study:

• To develop a computational pipeline for accurate identification of CRISPR arrays, distinguishing true CRISPRs from mimics.
• To investigate the prevalence, evolutionary origins, and potential functionality of CRISPR arrays located away from cas genes.

Main Methods:

• Development of a computational pipeline to filter false-positive CRISPR predictions.
• Comparative genomic analysis of CRISPR arrays and cas gene distribution in bacterial and archaeal genomes.
• Reconstruction of evolutionary events in closely related bacterial genomes to infer origins of isolated CRISPR arrays.

Main Results:

• A computational pipeline successfully eliminated false CRISPR predictions, revealing up to 25% of CRISPR arrays are isolated from cas genes.
• Isolated CRISPR arrays share repeat sequences with cas-adjacent arrays, suggesting evolutionary relatedness.
• Spacers from isolated arrays exhibit significant matches to viral genomes, indicating potential recent or ongoing functionality.

Conclusions:

• Isolated CRISPR arrays are a common feature in microbial genomes, linked evolutionarily to canonical CRISPR-Cas loci.
• Three primary evolutionary routes for isolated arrays include cas gene loss, de novo generation, and horizontal gene transfer.
• The emergence and integration of isolated arrays contribute to the functional diversification of CRISPR-Cas adaptive immunity.