A History of CRISPR
Francisco Mojica was the first researcher to characterize what is now called a CRISPR locus, reported in 1993. He worked on them throughout the 1990s, and in 2000, he recognized that what had been reported as disparate repeat sequences actually shared a common set of features, now known to be hallmarks of CRISPR sequences (he coined the term CRISPR through correspondence with Ruud Jansen, who first used the term in print in 2002). In 2005 he reported that these sequences matched snippets from the genomes of bacteriophage (Mojica et al., 2005). This finding led him to hypothesize, correctly, that CRISPR is an adaptive immune system. Another group, working independently, published similar findings around this same time (Pourcel et al., 2005)
Bolotin was studying the bacteria Streptococcus thermophilus, which had just been sequenced, revealing an unusual CRISPR locus (Bolotin et al., 2005). Although the CRISPR array was similar to previously reported systems, it lacked some of the known cas genes and instead contained novel cas genes, including one encoding a large protein they predicted to have nuclease activity, which is now known as Cas9. Furthermore, they noted that the spacers, which have homology to viral genes, all share a common sequence at one end. This sequence, the protospacer adjacent motif (PAM), is required for target recognition.
Koonin was studying clusters of orthologous groups of proteins by computational analysis and proposed a hypothetical scheme for CRISPR cascades as bacterial immune system based on inserts homologous to phage DNA in the natural spacer array, abandoning previous hypothesis that the Cas proteins might comprise a novel DNA repair system. (Makarova et al., 2006)
S. thermophilus is widely used in the dairy industry to make yogurt and cheese, and scientists at Danisco wanted to explore how it responds to phage attack, a common problem in industrial yogurt making. Horvath and colleagues showed experimentally that CRISPR systems are indeed an adaptive immune system: they integrate new phage DNA into the CRISPR array, which allows them to fight off the next wave of attacking phage (Barrangou et al., 2007). Furthermore, they showed that Cas9 is likely the only protein required for interference, the process by which the CRISPR system inactivates invading phage, details of which were not yet known.