Like humans, bacteria come under attack from viruses and rely on an immune system to defend themselves. A bacterial immune system known as CRISPR helps microbes “remember” the viruses they encounter and more easily fend them off in the future. Since researchers first discovered CRISPR in the mid-2000s, they have noticed something peculiar: It records confrontations with viruses sequentially, placing the most recent attack first in a series of genetically encoded memories. Now, two researchers at The Rockefeller University have explained why microbes store their immunological memories in this particular way. Their results were published online on September 8, 2016 in Molecular Cell. The article is titled “CRISPR-Cas Systems Optimize Their Immune Response by Specifying the Site of Spacer Integration.” “Until now, no one knew if this organizational feature serves a purpose, let alone what that might be,” says senior author Luciano Marraffini, an Associate Professor and Head of the Laboratory of Bacteriology. “We found an answer: It allows the microbe to mount the strongest immune response against its most recent threat, which is likely to be the most potent one around.” Microbial CRISPR systems remember viruses by capturing genetic snippets from them and storing them like beads on a string. (CRISPR stands for clustered regularly interspaced short palindromic repeats.) Should the cell meet a particular virus again, CRISPR-associated (Cas) enzymes use these snippets, known as spacers, to recognize and cut the virus. Thanks to this precision, one such system, CRISPR-Cas9, has become a powerful tool for researchers editing genomes. Researchers have wondered for some time why CRISPR systems create a chronological record of encounters with bacteria-attacking viruses known as phage. Dr.
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