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Many of the recently discovered bacterial antiviral systems are structurally and functionally homologous to components of metazoan innate immunity. An example of this ancestral link includes CBASS in bacteria, which resembles the metazoan mid-cyclic GMMP-AMP synthase (CGAS)-interferon gene stimulating factor (STING) antiviral pathway. CBAS contains two core components: the CGAS DNCV-like cyclic nucleotidyl transferase (CD-NTase, also known as CDN) enzyme, which produces cyclic nucleotides upon viral (bacteriophage) infection, and effector proteins that bind cyclic nucleotides to trigger host death or growth arrest, thereby inhibiting viral reproduction. In addition to cyclases and effector genes, CBASS operons can also encode accessory proteins, which are used to classify them into four main types, I-IV.
A central aspect of cyclic nucleotide immunity is the activation mechanism of cyclases, that is, how the enzyme senses a viral infection and begins to synthesize a second messenger. In the case of human CGAS, this is achieved by direct interaction with viral double-stranded DNA present in the cytoplasm. However, other CGAS homologs present in animals can sense RNA but not DNA. However, the mechanism of cyclase activation during bacterial CBASS response is not well understood.
Recently, biochemical analysis of bacterial cyclases from a variety of different CBAS operons has shown that some of these enzymes have constitutive activity in vitro, suggesting that their activity in vivo is negatively regulated and will only be released when recognized by phages. Conversely, there are also many examples of CBASS cyclase being inactivated in vitro, so an activation mechanism must be required to initiate the immune response. This study revealed the activation mechanism of type I cbass cyclase in staphylococcus.
The study was published in Nature under the title: "Bacterial cgas senses a viral RNA to initiate immunity".