Bacterial chemotaxis and the CheY signalling system

Verworn (1899) defined chemotaxis as the property of organisms capable of active movement by which they move toward or away from the source of a chemical stimulus, called positive and negative chemotaxis.¹ Such stimulation is unilateral only when concentration rises gradually in one direction from the organism.¹ Jennings (1906) worked with Paramecia and paired solutions. The animals gathered in the weaker solution because they avoided the stronger one, so one concentration could attract or repel depending on its partner.² Driesch (1908) reported that Jennings had shown chemotaxis to be the summed result of many single performances, not a simple orientation.³ Driesch added that Loeb did not accept resolving every kind of taxis in this way.³

Freddolino and colleagues (2018) describe bacterial chemotaxis as a biased random walk. Stochastic transitions in the rotation of the flagellar motor are biased toward the direction that raises chemoattractant signalling over time. They cite Macnab and Koshland (1972) for this, and note that later work revealed molecular mechanisms carrying out rather complex computations.⁶ Fields, Glazebrook and Levin (2021) give the CheY version. High phosphorylated CheY sends the motor counter-clockwise and the cell swims straight. Low phosphorylated CheY sends it clockwise and the cell tumbles.⁷ They state that the default ratio of phosphorylated to unphosphorylated CheY depends on the balance of kinases and phosphorylases, and fixes where behaviour switches.⁷

Matveev (2010) reviewed work by Kim and colleagues on the cytoplasmic domains of the E. coli chemotaxis receptor. Kim et al. concluded that an attractant lowers the number of secondary structures in the domain, which blocks signal transmission into the cytoplasm. A repellent has the opposite effect and makes signalling possible.⁴ Matveev read this through the hypothesis of native aggregation, in which a repellent puts the domain in an excited state. Kim and colleagues also held that methylation matters for clustering because it changes the amount of secondary structure.⁴ Grodstein, McMillen and Levin (2023) describe the feedback as integral-based. A receptor can be methylated in five places, and more methylation means more frequent tumbles.⁹

Lyon (2015) calls the idea that microbes know their world controversial but unoriginal. Jennings claimed that lower organisms, bacteria included, could inform the study of higher animals, and Koshland later made bacterial chemotaxis a model system for the study of behaviour.⁵ Timsit and Grégoire (2021) trace the analogy between cell signalling and neurobiology to Adler and Koshland. They credit Stock and collaborators in the early 2000s with the idea of molecular proto-brains, built from clusters of chemotactic receptors.⁸ Fields, Glazebrook and Levin call the CheY system a simple example of a biological quantum reference frame.⁷