A gated relaxation oscillator controls morphogenetic movements in bacteria

Dynamic control of cell polarity is of critical importance for many aspects of cellular development and motility. In Myxococcus xanthus, a G-protein and its cognate GTPase-activating protein establish a polarity axis that defines the direction of movement of the cell and which can be rapidly inverted by the Frz chemosensory system. Although vital for collective cell behaviours, how Frz triggers this switch has remained unknown. Here, we use genetics, imaging and mathematical modelling to show that Frz controls polarity reversals via a gated relaxation oscillator. FrzX, which we newly identify as the primary Frz output, provides the gating and thus acts as the trigger for reversals. Slow relocalisation of the polarity protein RomR then creates a refractory period during which another switch cannot be triggered. A secondary Frz output, FrzZ, decreases this delay allowing rapid reversals when required. This architecture thus results in a highly tunable switch that allows a wide range of motility responses.