Vibrio cholerae, the etiological agent of cholera, was found to be attracted by taurine (2-aminoethanesulfonic acid), a major constituent of human bile. Mlp37, the closest homolog of the previously identified amino acid chemoreceptor Mlp24, was found to mediate taxis to taurine as well as L-serine, L-alanine, L-arginine, and other amino acids. Methylation of Mlp37 was enhanced upon the addition of taurine and amino acids. Isothermal titration calorimetry demonstrated that a purified periplasmic fragment of Mlp37 binds directly to taurine, L-serine, L-alanine and L-arginine. Crystal structures of the periplamic domain of Mlp37 revealed that L-serine and taurine bind to the membrane-distal PAS domain in essentially in the same way. The structural information was supported by characterising the in vivo properties of alanine-substituted mutant forms of Mlp37. The fact that the ligand-binding domain of the L-serine complex had a small opening, which would accommodate a larger R group, accounts for the broad ligand specificity of Mlp37 and allowed us to visualise ligand binding to Mlp37 with fluorescently labelled L-serine. Taken together, we conclude that Mlp37 serves as the major chemoreceptor for taurine and various amino acids.

In a highly subcritical system with reflector, it is demonstrated that none of the methods so far developed serves usefully in determining the value of the multiplication factor with good accuracy, and that the fundamental difference between the essential properties of the prompt and the delayed neutron modes makes it to be extremely difficult to develop a new method to fulfil this need. Based on this judgement, it is proposed to abandon the widely used concept of multiplication factor, and to look for a new concept to represent the state of neutron multiplication, in the case of reflected highly subcritical systems. In this paper the authors present a new concept in which a fundamental prompt neutron decay constant a 000 is adopted to measure the neutron multiplication, this constant being of such nature as to satisfy the following two fundamental conditions: (1) It is uniquely measurable free from spatial and energetic dependence, and (2) it is amenable to direct calculation from the fundamental equation for neutron transportation.

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