RNA-Focused Defense: Degradation of Phage Transcripts by CRISPR-Associated RNases Enables Type III CRISPR-Cas Bacterial Immunity Against Viruses

When seeking to protect themselves from viruses, some bacteria use a seemingly risky strategy: They wait until the invading virus has already begun to replicate. Recent research at The Rockefeller University has shown how the microbes use two newly identified enzymes to fight off an infection even after delaying initial action. “A viral infection can kill a bacterial cell—or in some cases, the viral genetic material can provide benefits, such as protection against other viruses. Harmful viruses immediately begin replicating, but beneficial ones implant themselves into the bacterial genome,” says Luciano Marraffini (at left in photo), Ph.D., Assistant Professor and Head of Rockefeller’s Laboratory of Bacteriology. “By using a wait-and-see approach, and tolerating the initial phase of the infection, the bacteria are able to make an intelligent choice.” The new research, published online on February 4, 2016 in Cell, helps explain how bacteria manage to clear a harmful infection in spite of their slow response. In the future, it might inform the development of new ways to combat infectious disease, among other potential applications. The Cell article is titled “Degradation of Phage Transcripts by CRISPR-Associated RNases Enables Type III CRISPR-Cas Immunity.” The new study zeroes in on two enzymes, Csm3 and Csm6, which are part of a bacterial immune system known as CRISPR-Cas. When the Csm3 and Csm6 enzymes swing into action during the late phase of infection, they act to cut up viral RNA. The CRISPR (clustered regularly interspersed short palindromic repeats) system is a type of adaptive bacterial immune response that relies on sections of DNA containing sequences matching those in viral genetic codes. CRISPR-associated genes use these sequences as guides to target invaders for destruction.
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