Long as I can see the light – Control of cyanobacterial phototaxis orientation by the Tax1 system

In order to seek out optimal photosynthetic conditions, cyanobacteria exhibit phototaxis, navigating either towards or away from a light source via polar activation of type IV pili (T4P). The Tax1 system is a chemotaxis-like signal transduction pathway that governs a switch in cell polarity, which is crucial for positive phototaxis in Synechocystis sp. PCC 6803. The system consists of a blue/green light sensor, PixJ, which controls the histidine kinase PixL and two CheY-like response regulators, PixG and PixH. However, the molecular mechanisms by which Tax1 regulates polar pilus assembly and thereby phototactic orientation are poorly understood. Here, we investigated the phosphotransfer between PixL and its cognate response regulators in vitro and analyzed the localization and function of wild-type and phosphorylation-deficient PixG and PixH during phototactic motility. We demonstrated that both PixG and PixH are phosphorylated by PixL but have different roles in the regulation of phototaxis. Only phosphorylated PixG interacts with the T4P motor protein PilB1 and localizes to the leading cell pole under directional light, thereby promoting positive phototaxis. In contrast, PixH acts as a phosphate sink, reducing PixG phosphorylation and inhibiting positive phototaxis. In addition, we showed that the C-terminal receiver domain of PixL is essential for positive phototaxis, and modulates the kinase activity of PixL. Our findings reveal the molecular basis of positive phototaxis regulation by the Tax1 system and provide insights into the division of labor between PatA-type and CheY-like response regulators in cyanobacterial chemotaxis-like systems. Furthermore, these findings highlight similarities in the regulation of movement direction during twitching motility in phototactic and chemotactic bacteria.