Many species of bacteria – including those that infect humans – are themselves threatened by parasites and predators. A large group of viruses called bacteriophages are known to infect bacteria. On infection, the phage transfers its DNA to the bacterial cell, hijacks the bacterial DNA-replicating machinery to make multiple copies of itself, and then escapes by killing the bacterial cell.
Bacteria have in turn evolved a variety of immune mechanisms to protect themselves against invading phages. About 40% of sequenced bacterial genomes contain a set of CRISPR/Cas genes. These include a set of genes encoding Cas proteins as well as CRISPR loci, which are arrays of short repeats separated by highly variable ‘spacer’ sequences.
These spacer sequences are identical to sequences present in phage DNA. They are transcribed into small RNA molecules called crRNAs, which, with the help of the Cas proteins, bind to and cause degradation of the invading phage DNA. However, a recent study published in Nature has discovered a novel CRISPR/Cas system – not in a bacterium, but in a bacteriophage.
This bacteriophage, called ICP1, attacks a strain of the cholera pathogen Vibrio cholerae. The authors demonstrated that the CRISPR/Cas system in the bacteriophage is fully functional and targets a region of bacterial DNA that is responsible for defense against the phage. Not only is this an example of a supposedly bacterial immune mechanism being used by a phage, but it appears that the phage uses it to counter an entirely different bacterial immune mechanism. This host vs. pathogen arms race suddenly looks a lot more interesting.