The Calp (CRISPR associated Lon protease) system elicits a transcriptional response to infection when CalpL (Calp Lon protease) binds cA4 in its SAVED (SMODS associated and fused to various effectors domain) sensor domain, resulting in filament formation and activation of the Lon protease domain, which cleaves the anti-Sigma factor CalpT, releasing the CalpS (Calp Sigma factor) for transcriptional remodelling. Here, CA4 is linked to infection.