In gram-negative bacteria, the extracellular proteins have two lipid bilayers to cross, in contrast to the proteins targeted to the periplasm or the outer membrane. Depending upon the secretion route and the genes involved, two major secretion pathways can be identified. Without traversing the periplasm, oc-hemolysin of Escherichia coli is secreted through a hemolysin translocator composed of HlyB, HlyD (24), and ToiC (48). Two genes, prtD and prtE, required for the secretion of an extracellular protease by Erwinia chrysanthemi were found to be homologous to hlyB and hlyD, respectively (27). In the second category, extracellular proteins, for example, the pullulanase of Klebsiella oxytoca, are secreted via the periplasm (34). A closely linked operon includes at least 12 pul genes encoding proteins required for the secretion of pullulanase in addition to the sec genes involved in the translocation of proteins across the inner membrane (34, 35). Homologs of the pul genes are also required for the secretion of extracellular proteins by E. chrysanthemi (22), Pseudomonas aeruginosa (15), Xanthomonas campestns pv. campestris (12), and Yersinia enterocolitica (29). Mutations in these genes can result in the accumulation of extracellular proteins in the periplasm (1,7,11,16,29,34,45,49
The relative spatial position of each chain link and the phase angle of engaged tooth are not concerned in traditional sprocket designs. As such, the chain shifting might fail regularly. In this study, an optimized path of one chain for up-shifting is derived. Such path is formed by several bended chain links sustain yaw and roll. Empirically, appropriate bending, yaw and torsion angles for tooth chamfer, could be shorten the up-shifting distance and phase angle required, eventually facilitate not only more ascending points compactly arranged on sprocket but larger chain laterodeviation which curtails sprockets clearance and integrates the sprocket unit. Then again, the optimized shifting path promotes efficiency and decreases required movement yielded by derailleur tappet pressure.
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