Study On Chemotaxis,Motile Behavior And Non-equilibrium Model Of Molecular Motor Of E.coli

Bacteria is the first form of life on Earth,and a lot of researches have been done since the discovery centuries ago.There exist a wide variety of bacteria,and different types of bacteria show very different behavior.The relationship between bacteria and human being is very close.Human life and health are closely related to bacteria,such as wound infections,alcohol brewing,some diseases and industrial production and so on.It is of great significance to study bacteria for the improvement of human living conditions and manufacture of industrial products.As one of the simpliest forms of life,the behaviors of bacteria reflect the physiological mechanism of higher organisms.As a consequence,the study of bacterial behavior and biochemical phenomena at molecular level also plays a rather important role in understanding general biological phenomena.E.coli(Escherichia coli)is a common model organism in bacterial and biochemical researches,which lives in the lower intestine of warm-blooded animals.As a member of the large family of bacteria,its structure and behavior is simple and complex.E.coli can respond to external stimulus,and takes different movement strategies according to the stimuli it senses,called chemotaxis.The flagella motor which anchored on cell wall is a very extraordinary and sophiscated protein molecular machine.The pathogenicity of E.coli is closely related to its chemotaxis and movement of flagella motor.Many species of bacteria also have similar behavior and structure,so the study of E.coli will help people develop drugs against bacterial infections and help with the utilization of bacteria.In this thesis,we used E.coli as the research object to study its chemotaxis singnal transduction network,motile behavior and flagella motor.Bacteria sense and respond accordingly to external environment through the chemotaxis signal transduction network.Once phosphorylated,the CheY protein in the network can change the probability of rotation direction of the flagella motor,leading to the regulation of movement behavior.A few studies have shown the existence of noise in the network,which is the random fluctuation of phosphorylated CheY concentration over time,and the noise is essential in coordination of multiple motors in a bacterium and can eahance bacterial drift velocity in chemical gradients.In this thesis,we measured the magnifitude of this noise in wild-type cells by compraring the behavioral difference between wild-type E.coli and mutant without signal noise,and found that the noise increases the sensitivity of the bacterial chemotaxis network downstream at the level of the flagellar motor.This provided a simple mechanism for the noise-induced enhancement of chemotactic drift,and we confirmed this by simulating the E.coli chemotactic motion in the environment with different gradients of chemo-attractant.The chemotaxis signal transduction network of bacteria eventually affects their motile behaviors.Bacteria can better survive and multiply in the complex and changing enviroment through the adjustment of motile behaviors.The study of bacterial motility has always been an important direction.Most bacteria in nature live in complex three dimensional enviroments,so the observation of three dimensional movement of bacteria is of more practical significance.There are several three-dimensional tracking technologies for bacteria,each has its own advantages and disadvantages.We reconstruct the three-dimensional trajectories of E.coli in a simple way by combining defocused particle tracking technique and the dark field microscopy on an ordinary optical microscope.We compared the behavioral differences between wild-type E.coli and mutant without signal noise.We confirmed the feasibility of this method by analysis of the three dimensional motile behavior of wild-type E.coli.Comparison of the motility behaviors of the two strains showed that the tumble frequency of wild-type strain decreased with the same average level of phosphorylated CheY.We also found that the run length distribution showed a non-exponential shape which is different from former study,which deserves further research.The movement of E.coli or other flagellated bacteria is driven by the flagella motor on cell body.The switching mechanism of the flagellar motor provides the basis for the motile behaviour of flagellated bacteria.The molecular motor is very sensitive to the level of phosphorylated CheY.Previous study have explained the phenomena well by the equilibrium model,such as the two-state concerted allosteric model or the Ising-type conformation spread model.In this paper,we introduced the non-equilibrium factor caused by the torque into the motor model based on the experimental results and the conformational spread model.Through the introduction of non-equilibrium factor,the residence time distributions of the motor under different experimental conditions can be reproduced.Furthermore we found that this non-equilibrium factor increases the sensitivity of flagellar switch.Exploiting a very small fraction of the energy expense of the flagellar motor for functional regulation increases its sensitivity greatly.