A Phage Ejectosome Protein Moonlights to Inhibit CBASS Anti-Viral Defense System

A family of prokaryotic antiviral systems collectively known as the cyclic oligonucleotide-based signaling system (CBASS) is found in many bacterial species. Phage defense in CBASS is mediated by the action of a specific effector activated by the cognate cyclic oligonucleotide, often resulting in abortive infection. In the prototypical Type II-A CBASS, phage defense is initiated by the production of cyclic GMP-AMP (cGAMP) by enzymes known as cGAS/DncV-like nucleotidyltransferases (CD-NTases) within the bacterial host, and subsequent activation of the CapV phospholipase by cGAMP leads to cell lysis, thereby limiting phage replication within the host population. Phage proteins that degrade cGAMP, sequester cGAMP, or prevent cGAMP synthesis have been identified in phages to evade CBASS protection. However, some phages are resistant to CBASS protection despite not encoding these proteins, suggesting that additional counter-defense mechanisms exist. To identify new anti-CBASS mechanisms in phages, we exploited bacterial host sensitivity to the antimicrobial compound sulfamethoxazole (SMX) as an indirect read-out for CBASS activity. Bacteria with active CBASS are more sensitive to SMX; therefore, we postulated that any viral proteins produced inside the bacterial host that target CBASS could increase bacterial SMX resistance. Using this screen, we identified that the Gp15 protein from the vibriophage ICP3 increases SMX resistance in Vibrio cholerae El Tor biotype that encodes an active CBASS. Gp15 is an essential virion protein involved in DNA ejection and conserved in many phages. Expression of Gp15 of another coliphage increases SMX resistance in Escherichia coli expressing an active CBASS, and it allows CBASS-sensitive phage to infect the bacterial host with CBASS, demonstrating an important role of Gp15 in phage defense evasion. Unlike other known anti-CBASS proteins, Gp15 does not degrade or sequester cGAMP, and it does not reduce the cellular level of cGAMP. Gp15 also does not interfere with CapV binding to cGAMP. Instead, Gp15 directly inhibits cGAMP-activated CapV serine hydrolase activity in a stoichiometric manner. Together, our study reveals a new mechanism for CBASS antagonism, in which a phage DNA ejection protein moonlights to inhibit a CBASS effector, thereby evading anti-phage defense.