The development of immunological memory in bacteria 

CRISPR mediated bacterial immunity, with permission
from NPG, from Yosef I et al, Nature 2015

Two papers published last week in Nature describe the amazing development of bacterial immunological memory. Previously it has been thought that in contrast to vertebrates, bacteria can not remember previous infections. Now it is obvious that the original function of CRISPR is to fight against invading viruses by cleaving specific parts of the viral genome. During an infection, part of the viral genome is stored in the bacterial genome as a library, from which a matching sequence can be utilized upon subsequent infection. How this happens remained elusive until recently, now it is shown that components of the CRISPR system (Cas1, Cas2, Csn1, Cas9 and tracrRNA) act in concert to find appropriate target regions in the viral genome, and insert a copy into the bacterial genome. Finally it makes sense, why there is a so-called PAM site. This is a very short motif adjacent to the recognition region, which is an absolute sequence requirement of the system (e.g. this is NGG for the S. pyogenes CRISPR system). Most importantly, when pasted into the bacterial genome, there is no PAM site next to the sequence. So that is why bacteria do not attack themselves, since the PAM site is only in the virus, leading to cleavage. That is how bacteria evade autoimmunity. Smart!

Check out the original papers here: http://goo.gl/kz2X1V