Separating Functions of the Phage-Encoded Quorum-Sensing-Activated Antirepressor Qtip.

Quorum sensing is a process of chemical communication that bacteria use to track cell density and coordinate gene expression across a population. Bacteria-infecting viruses, called phages, can encode quorum-sensing components that enable them to integrate host cell density information into the lysis-lysogeny decision. Vibriophage VP882 is one such phage, and activation of its quorum-sensing pathway leads to the production of an antirepressor called Qtip. Qtip interferes with the prophage repressor (cI), leading to host-cell lysis. Here, we show that Qtip interacts with the N terminus of cI, inhibiting both cI DNA binding and cI autoproteolysis. Qtip also sequesters cI, localizing it to the poles. Qtip can localize to the poles independently of cI. Alanine-scanning mutagenesis of Qtip shows that its localization and interference with cI activities are separable. Comparison of Qtip to a canonical phage antirepressor reveals that despite both proteins interacting with their partner repressors, only Qtip drives polar localization.