Team:Peking/Project/Phototaxis

Introduction

Bacterial phototaxis refers to the ability of bacteria to sense light changes in their extracellular environment and to bias their motility towards or away from the light. Phototactic responses are observed in many organisms such as Serratia marcescens, Tetrahymena, and Euglena. The behavior of phototaxis is the directed movement up a gradient to an increasing amount of light. This is analogous to positive chemotaxis; the usage of light as the attractant rather than chemicals.

Figure 1.

For synthetic biology, phototaxis is a palace of mysteries because it has a rapid, precise response in comparison with chemotaxis while its mechanism has not been fully elucidated so far. As chemical diffused in the culture medium, chemotaxis has not the preciseness to origin of attractants. On the contrary, light is noninvasive, reversible and of good spatiotemparol control. With these advantages, phototaxis is more qualified for the application than chemotaxis in synthetic biology. Also, light signal is a potential link between the electrical components and bio-elements that endows phototaxis with a great potential of technology improvement such as light-guided cell targeting for medicine aggregation or tissue recovery in biomedical applications, light-induced bacterial enrichment or dispersion in environment protection and so forth.(Fig 1)

Previous research confirmed that at least four types of photosensory proteins are involved in prokaryotic phototaxis: the sensory rhodopsins in halophilic archea, PYP in Halorhodospira halophila, and the BLUF protein PixeD and phytochrome-related proteins PixJ1 and Cph2 in Synechocystis. But all of above left some part unknown such as protein structure or downstream locomotive machinery so that they can’t be applied to the design of bacterial phototaxis . To implement phototaxis, we conceive that it is possible to achieve our goals by combining our luminesensor with certain chemotacic protein. (Fig 2)

Figure 2. Distribution of photoreceptor proteins in the three domains of life. These photoreceptor proteins of six well characterized photoreceptor families function in nature to mediate light induced signal transduction. The phytochromes can be found in bacteria, fungi, and plants.

The model of chemotaxis pathway is quite clear and widely accepted. E.coli alternates between run(smooth swimming) and tumble, which result from distinct types of flagellar rotation, counter-clockwise(CCW) and clockwise(CW), respectively. A number of intracellular proteins provide the necessary signaling cascade which links the membrane receptors to the flagellar: CheY,CheZ,CheW, CheA,(Fig 3) etc.

Figure 3. The simplified scheme of protein-protein interactions during chemotaxis of bacteria.

Among those proteins, CheY have the direct attachment to the flagella motors. Phosphorylated CheY diffuses through the cytoplasm to the motors，binding to FliM, a component of the C ring of the flagellar motor, and induces CW rotation of the motor. CheZ can dephosphorylate CheY, resulting in CCW rotations which leads to the smooth swimming. CheZ mutants result into swarming while CheY mutants are smooth-swimming.

CheZ is commonly used to affect bacterial motion in synthetic biology. Experiments shows that different levels of CheZ induced by arabinose can change the diameter of the swarming colony, combination between the quorum sensing part and CheZ can make bacteria form a stripe pattern. So we suppose combination between CheZ and our Luminesensor must have an effect on bacteria in motion. As modeling reveals, it does have a phototaxis phenomenon where light is a negative stimuli, showing that it is very promising to build a ultra-sensitive, effective, promising phototaxis design.