Left: SgrAI dimers (magenta) bind to primary (green boxes) and secondary (grey boxes) sequences on both the invading phage DNA and the host DNA. Primary sequences on the host DNA are methylated and are not bound by SgrAI. SgrAI largely maintains the T state prior to filamentation (pink ellipses). Center: SgrAI bound to the primary recognition sequence occupies the R state (blue ellipses) in sufficient concentrations to nucleate filaments drawing in and activating SgrAI bound to secondary sites on the phage DNA. SgrAI bound to secondary sequences on the host genome do not induce filamentation and are not drawn into filaments nucleated by SgrAI bound to the phage DNA because of the slow, second-order rate constant for filament assembly and the low concentrations of SgrAI bound DNA in the cell when on separate DNA molecules. Right: SgrAI cleaves primary and secondary sequences in the phage DNA only, leaving the host DNA unharmed (red arrow).