Figure 3.

Targeting secondary messenger. The use of second messengers to signal viral invasion has been reported in various prokaryotic systems, including CBASS (A), CRISPR-Cas system (B), and Thoeris system (C). Phage-encoded proteins can degrade second messengers in an enzymatically active form or bind and sequester them in a non-enzymatically active form without cleaving small molecules. Pyrimidine cyclase systems, referred to as Pycsar, exhibit analogous functionality to the CBASS mechanism. The anti-Pycsar protein Apyc1 and anti-CBASS protein Acb1 act as inhibitors of anti-phage defense through the cleavage of second messengers, demonstrating a similar mechanism. For reference, please refer to Figure 3A for the mechanism models of both Pycsar and Apyc1.