COMPUTER-AIDED RESOLUTION OF AN EXPERIMENTAL PARADOX IN BACTERIAL CHEMOTAXIS

Escherichia coli responds to its environment by means of a network of intracellular reactions which process signals from membrane-bound rece ptors and relay them to the flagellar motors, Although characterizatio n of the reactions in the chemotaxis signaling pathway is sufficiently complete to construct computer simulations that predict the phenotype s of mutant strains with a high degree of accuracy, two previous exper imental investigations of the activity remaining upon genetic deletion of multiple signaling components yielded several contradictory result s (M. P. Conley, A. J. Wolfe, D. F. Blair, and H. C. Berg, J. Bacterio l. 171:5190-5193, 1989; J D. Liu and J. S. Parkinson, Proc. Natl. Acad . Sci. USA 86:8703-8707, 1989). For example, ''building up'' the pathw ay by adding back CheA and CheY to a gutted strain lacking chemotaxis genes resulted in counterclockwise flagellar rotation whereas ''breaki ng down'' the pathway by deleting chemotaxis genes except cheA and che Y resulted in alternating episodes of clockwise and counterclockwise f lagellar rotation. Our computer simulation predicts that trace amounts of CheZ expressed in the gutted strain could account for this differe nce. We tested this explanation experimentally by constructing a mutan t containing a new deletion of the che genes that cannot express CheZ and verified that the behavior of strains built up from the new deleti on does in fact conform to both the phenotypes observed for breakdown strains and computer-generated predictions, Our findings consolidate t he present view of the chemotaxis signaling pathway and highlight the utility of molecularly based computer models in the analysis of comple x biochemical networks.