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Characterization and evolution of Salmonella CRISPR-Cas systems
Nikki Shariat1, Ruth E Timme2, James B Pettengill2
1Department of Food Science, Center for Molecular Immunology and Infectious Disease and Center for Infectious Disease Dynamics, Pennsylvania State University, University Park, PA 16802, USA.
Microbiology (Reading, England)
|December 7, 2014
Summary
Salmonella CRISPR-Cas systems, crucial for bacterial immunity, are highly conserved but no longer adaptive. Their genetic integrity suggests a potential alternative function beyond defense against invasive elements.
Area of Science:
- Microbiology and Molecular Biology
- Bacterial Genetics and Evolution
- CRISPR-Cas Systems
Background:
- Prokaryotic CRISPR-Cas systems confer adaptive immunity against foreign genetic elements.
- These systems comprise cas genes, CRISPR arrays (spacers and direct repeats), and leader sequences.
Purpose of the Study:
- To investigate the CRISPR-Cas systems in over 600 Salmonella isolates across four serovars.
- To analyze the conservation, composition, and evolutionary dynamics of Salmonella CRISPR-Cas loci.
Main Methods:
- In-depth sequence analysis of CRISPR-Cas system components (cas genes, CRISPR arrays, leader sequences).
- Comparative genomics across multiple Salmonella serovars and isolates.
- Analysis of spacer content for matches to exogenous nucleic acids and self-targeting sequences.
Main Results:
- CRISPR-Cas features are highly conserved in Salmonella, with CRISPR1 being more conserved than CRISPR2.
- Array composition is serovar-specific, but evidence for recent spacer acquisition is lacking.
- High nucleotide identity in cas operons suggests historical immune activity and cessation of adaptive events.
Conclusions:
- Salmonella CRISPR-Cas systems appear to be evolutionarily inactive as immune systems.
- Remarkable conservation of these loci across divergent serovars challenges the notion of degeneration.
- The conserved nature suggests a potential alternative, non-immunogenic function for these systems in Salmonella.
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