Identification and structural analysis of the tripartite α-pore forming toxin of Aeromonas hydrophila.

The alpha helical CytolysinA family of pore forming toxins (α-PFT) contains single, two, and three component members. Structures of the single component Eschericia coli ClyA and the two component Yersinia enterolytica YaxAB show both undergo conformational changes from soluble to pore forms, and oligomerization to produce the active pore. Here we identify tripartite α-PFTs in pathogenic Gram negative bacteria, including Aeromonas hydrophila (AhlABC). We show that the AhlABC toxin requires all three components for maximal cell lysis. We present structures of pore components which describe a bi-fold hinge mechanism for soluble to pore transition in AhlB and a contrasting tetrameric assembly employed by soluble AhlC to hide their hydrophobic membrane associated residues. We propose a model of pore assembly where the AhlC tetramer dissociates, binds a single membrane leaflet, recruits AhlB promoting soluble to pore transition, prior to AhlA binding to form the active hydrophilic lined pore.

Initial and concurrent planning in solutions to well-structured problems.

Two experiments are reported, which consider planning behaviour in the context of a well-structured problem. One question in the problem-solving literature is to what extent planning a solution to a problem takes place before attempting that problem and whether this takes precedence over planning while solving a problem, hereafter referred to as concurrent planning. An additional question is whether the adoption of one mode of planning confers a performance advantage and under what circumstances one strategy is adopted in preference to others. The studies reported here set out to investigate the effects on performance of adopting different modes of planning and whether there is any relationship between the adoption of different strategic approaches and problem-solving performance. The results of these studies suggest that initial planning can enhance problem-solving performance, but only when problems remain relatively simple. As problem complexity increases the effects of initial planning appear to have little or no effect upon performance. In conclusion it is suggested that strategy use depends upon the interactions between individual preference for a given strategy, problem complexity, and the stage that one has reached in the development of a solution to a problem.