Bacteria have an ability to sense a change in their environment and respond to it by changing their swimming pattern. In such behavior, the mechanism of chemotaxis has been extensively elucidated. Chemical stimuli are at first received by the receptor located on the surface of periplasmic site of the membrane. Then, the information is transferred into cytoplasmic site of membrane through the transducer proteins located in the membrane. Signals are produced from the transducers and transmitted to the switches of flagellar motor by a concerted work of various proteins required for chemotaxis coded by so-called che genes. In this review, we mainly describe the molecular nature of transducer proteins, since many data concerning the transducers have been accumulated by using the genetic engineering techniques.
The mechanism of protein folding is presented, which can account for the thermodynamical phase transition and the rapidity of folding process. The long-range feature of hydrophobic interaction and its specificity after the formation of the secondary structures play the essential role in folding. The refoldings of flavodoxin, erabutoxin and BPTI (bovine pancreatic trypsin inhibitor) are discussed. Finally a possible mechanism of folding in membrane proteins is proposed, and applied to bacteriorhodopsin.