A biological system must consume a lot of energy to establish and maintain its well- organized structures and functions, and avoid complete disorder and death. In this respect, even the tiny bacterial cells should also possess some kind of intracellular organization, and cannot be simple "bags of enzymes". In fact, during the past decade, numerous researches have demonstrated that bacteria also have highly complex intracellular structural networks, like their eukaryotic counterpa rts (Gitai, 2005; Shih & Rothfield, 2006). Thus, all living organisms, be it a multicellular animal or plant, or an unicellular microbe, possess different organs/organelles or primitive organelles (for example the membranous magnetosome in the magenetotactic bacteria, (Komeili et al, 2006)) to perform various functions with the energy uptaken from the environment. While unicellular organisms build spatially segregated organelles at the subcellular level, multicellular organisms take a further step to build different organs. Regardless of the wide difference in size between organisms of different phyla, the cell polarization mechanisms, which are responsible for the cell and organ differentiations, are often highly conserved (small G protein; cytoskeletons). This is proven to be true by the discove ry in this thesis work (Zhang et al, 2010).　　 Polarity is about asymmetric positioning: examples are the diffe rentiation of organs at the organismal level or the specific segreggation of subcellular organelles within one cell, which break the symmetry of the cell. Cell polarization is most obvious in eukaryotic cells, like the tree-shape neuron cells (Lee et al, 2006), brush-like epithelial cells (Fre et al, 2005), and the ever-changing shaped leukocytes (Campello et al, 2006). Polarization is generally essential for the physiological functions of a given cell type. For example, the tree like radial structure of neurons facilitates the transmission of the intercellular neurotransmitter (Yamada & Nelson, 2007); the brush-like epithelial cells increase the contact surface with digested food; and the ever-changing shape of leukocyte sharpens the instant response to the possible presence of antigens to chase and lyse them (lglesias & Devreotes, 2008). Consequently, defects in polarization cause severe diseases, such as cancer metastasis (Spaderna et al, 2008) and polycystic kidney disease (Fischer et al, 2006). Therefore, a vast amount of studies has focused on the molecular mechanisms of cell polarization in different eukaryotic cell types. Several factors responsible for cell polarization have been revealed, such as the self-organization of membrane lipids (Dustin, 2002; Horejsi, 2003; Kamiguchi, 2006; Manes & Viola, 2006), inherent and dynamic cytoskeletal elements (Li & Gundersen, 2008), and small GTPases and their cognate regulators (lden & Colla rd, 2008). 　　 In prokaryotes, cell polarity is not generally obvious from cell shapes (for rod and round- shaped bacteria). However, upon closer looking at the cells, even the symmetrical rod-shaped bacteria show some subcellular polarity. The most obvious examples are the various cell surface appendages, distributed at one cell pole (e.g. pili, flagellum, stalk) in various bacteria (Bahar et al, 2011; Canals et al, 2006; Kawagishi et al, 1995; Wagner & Brun, 2007). In fact, bacterial cells show an intricate internal organization, that is essential for their normal physiology, for example during chemotaxis with chemoreceptor complexes in Escherichia coli (Maddock & Shapiro, 1993; Skidmore et al, 2000) and cell division with the use of various cytoskeletal structures (Shjh & Rothfield, 2006). Over the last two decades, several polarization mechanisms have been described in prokaryotes. For example, membrane rafts were discovered to play important roles in protein positioning and signal transduction in prokaryotes (Arias-Cartin et al, 2011; Lopez & Kolter. 2010; Renner & Weibel, 2011). Geometric cues are also found to be responsible for the localization of sporulation related proteins such as SpoVM, and DivIVA in Bacillus subtilis (Lenarcic et al, 2009; Ramamurthi & Losick, 2009). Cell division also contributes to cell polarization by recruiting divisome complex favoring proteins at the newly born poles, such as the TipN protein in Cauloboaer crescentus (Huitema et al, 2006; Lam et al, 2006). Conversely, several proteins favor the relatively "old" cell pole, such as the ActA in Listerio monocytogenes and lcsA in Shigello flexneri (Goldberg & Theriot, 1995; Kocks et al, 1995; Smith et al, 1995). The discovery of homologs of eukaryotic cytoskeletal proteins in proka ryote prompted a new pulse to study bacterial internal structures and revealed that some proteins may use these structures to achieve localization, for example the polar DivJ kinase and PleC phosphatase that are essential for the asymmetrical life cycle progress in C. cresentus (Gitai et al, 2004). During sporulation of B. subtilis, chromosome gene polarity (gene position on chromosome) is employed to monitor and couple asymmetrical cell fate determination (Dworkin, 2003; Rocha, 2008). In some cases, some proteins, such as PopZ in C. crescentus, are capable of self-assembling into polymeric structures and become an organization center to direct the specific localization of proteins and DNA (Bowman et al, 2008; Ebersbach et al, 2008). Finally, a Ras-Iike small GTPase and its associated GAP protein polarize Myxococcus xanthus cells during motility (Leonardy et al, 2010; Zhang et al, 2010). Therefore, prokaryotes also evolved all set of polarization elements that exist in their euka ryotic counterparts.　　 Polarization mechanisms are generally coupled to sensing, allowing rapid and adapted responses to the ever changing environmental conditions. However, the sensory signal pathways involved in this process are quite different in eukaryotes and prokaryotes. In eukaryotes, G-protein coupled receptors (GPCRs) constitute the majority of the sensory pathway for cells to perceive environmental changes (Marinissen & Gutkind, 2001; Pierce et al, 2002). When bound with certain ligands, GPCRs activate the coupled small G proteins, which subsequently elicit dive rse cellular responses, including va riation of gene expression and/o r cell translocation (Dorsam & Gutkind, 2007; Ritter & Hall, 2009). Prokaryotes, on the other hand, evolved widely spread two-component systems to sense and respond to environmental cues (Stock et al, 2000; Wuichet et al, 2010). The first component, the sensory Histidine Kinase (HK), usually membrane bound, senses the exterior signals, and get auto-phosphorylated; the high energy phosphoryl group is then subsequently transmitted to a cognate Response Regulator (RR) protein, switching this protein on and affecting gene expression or protein- protein interactions (Stock et al, 2000). As will be discussed in this introduction, two- component signaling is used to dictate cell pola rity in bacteria (Biondi et al, 2006; Curtis & Brun, 2010; fniesta et al, 2006; Jacobs et al, 1999).　　 During this thesis work, we employed the modelbacteria Myxococcus xonthus to investigate the dynamic spatial regulation of its motility, which is absolutely critical to understanding its multicellular behaviors under both vegetative and developmental conditions We found, for the first time, that a prokaryote M. xanthus, like its eukaryotic counterparts, employs a small GTPase protein MgIA and its cognate GTPase activity activating protein (GAP) MgIB to establish the polarity of the motility systems (Leonardy et al, 2010; Zhang et al, 2010). This eukaryotic polarity module is invertible and responds to a specialized two-component signaling pathway. Thus, this thesis describes a new and potentially widespread mechanism of pola rity regulation in bacteria.