Fig. 7.

Proposed two step model for the Pgl mechanism by modification and then restriction. In uninfected cells Pgl proteins assemble into a ‘resting complex’ in which the toxic activity of PglX is inhibited by an interaction between PglX and PglZ. Following infection the resting complex changes (presumably triggered by a signal that is specific for phage ϕC31 and its relatives) to become either a DNA modifying complex or a restricting complex, dependent on whether the incoming phage is unmodified or modified, respectively. The modifying complex is likely to have N⁶-adenine methylation activity through the activity of PglX (blue). We propose that the restricting complex is activated by infection by modified phage. Candidate Pgl proteins that could mediate restriction are indicated in red. PglW^P and PglZ^P are putatively phosphorylated isoforms due to the kinase activity of PglW that transduce a signal to the cells of phage infection (more details are provided in the text).