A CRISPR revolution: How bacteria fight their own infections
Oeiras, 01.04.2014
Bacteria are able to become immune to bacteriophages (bacterial virus) by inserting pieces of the viral genome at specific locations of their own chromosome. These new chromosome segments - called CRISPR - are able, via a small RNA molecule, to recognize and target the inactivation of invading bacteriophages, a process mediated by specific proteins. A collaborative effort between the Control of Gene Expression Lab and other research groups from France has now identified a new type of CRISPR element that involves a completely different pathway to become active. The findings were published in PLoS Genetics.
Bacterial non-coding RNAs are key regulatory molecules of metabolic, physiologic and pathogenic process and include a novel class named CRISPR (Clustered Regulatory Interspaced Short Palindromic Repeats) shown to mediate bacterial adaptative immunity against bacteriophages. Approximately 40% of bacterial genomes contain at least one CRISPR locus. Upon transcription and RNA processing, mature CRISPR RNA molecules serve as guides for the sequence specific recognition of the foreign invader. The mechanism of action of all known CRISPRs requires specific proteins called Cas, which are able to cleave DNA. Based on this knowledge, Cas-based systems can be exploited as genetic tools: by designing RNAs that target the DNA of interest for cleavage by the Cas proteins, it is possible to construct mutants from bacteria to man and control gene expression. "It's really exciting what these molecules can do", says Cecilia Arraiano, who coordinated the study at ITQB, "In fact, engineered CRISPR-Cas systems are already revolutionizing genetic manipulation by providing game-changing tools for biotechnology".
Working with the pathogenic bacteria Listeria monocytogenes, researchers identified a new type of CRISPR element (called RliB) in the bacterial genome. The RliB-CRISPR is present in the same locus in all Listeria genomes but surprisingly, and unlike other CRISPR-systems, is never associated with a set of cas genes. Instead, researchers found that the nuclease PNPase, well known to the ITQB team, is the critical enzyme for RliB-CRISPR processing and activity. This adds one more function, thus far unknown, to this bacterial nuclease.
Listeria is a food-borne pathogen for humans and birds, causing a serious disease known as listeriosis. In the last few years, the bacteria Listeria monocytogenes has become a model organism for intracellular infection and an attractive system for the study of regulatory RNAs in pathogenic bacteria. Besides conferring immunity against bacteriophages, the RliB-CRISPR seems to be important for the pathogenesis of Listeria itself: researchers found that RliB-CRISPR was more expressed when the bacteria were grown in human blood and that this element is important for virulence. This highlights an important cross-talk between the bacteriophage and the pathogenic bacteria during the infection of mammalian cells.
Original Article
Plos Genetics DOI: 10.1371/journal.pgen.1004065
A PNPase Dependent CRISPR System in Listeria
Nina Sesto, Marie Touchon, José Marques Andrade, Jiro Kondo, Eduardo P. C. Rocha, Cecilia Maria Arraiano, Cristel Archambaud, Éric Westhof, Pascale Romby, Pascale Cossart.
