@@ -26,12 +26,10 @@ Bacteria can hide receptors behind surface structures such as extracellular poly
Bacteria encode a variety of defense systems that prevent phage infection from progressing in various ways. Despite all this variability, all bacterial defense systems are schematically composed of three parts: a sensor recognizing the infection, an effector that achieves protection and a way to transmit the information between the sensor and the effector, either through signaling molecules or protein-protein interactions. Phage anti-defense proteins can target all three of these components.
- Sensor targeting:
- Competitive binding to the sensor: an anti-DSR2 protein from phages phi3T and SPbeta can bind the bacterial DSR2 protein and prevent the physical interaction between DSR2 and its phage activator, the tail tube protein :ref{doi=10.1038/s41564-022-01207-8}. Moreover, Ocr protein from T7 can mimic a B-form DNA oligo and acts as a competitive inhibitor of bacterial type I restriction modification systems :ref{doi=10.1016/s1097-2765(02)00435-5}.
- Masking the activator: some jumbo phages are able to produce a nucleus-like proteinaceous structure that hides phage DNA and replication machinery away from DNA-targeted systems such as type I CRISPR system :ref{doi=10.1038/s41564-019-0612-5}.
- Transmission targeting:
- Degradation of signaling molecules: many systems rely on the production of a nucleotidic signaling molecule after phage sensing to activate the effector such as Pycsar, CBASS, and Thoeris systems. Phages possess proteins that can degrade these molecules to prevent effector activation, such as the anti CBASS Acb1 from phage T4 and the anti Pycsar Apyc1 from phage SBSphiJ :ref{doi=10.1038/s41586-022-04716-y}.
- Sequestration of signaling molecules: an alternative strategy is to bind the signaling molecule very tightly without degrading it, which still prevents effector activation but is presumably easier to evolve than a catalysis-dependent degradation. These phage proteins are called sponges, and two were identified as anti-Thoeris: Tad1 from phage SBSphiJ7 and Tad2 from phage SPO1 and SPO1L3 :ref{doi=10.1038/s41586-022-05375-9,10.1038/s41586-023-06869-w}.
