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Addition of micromolar concentrations of di-2-ethylhexyl phthalic acid ester (DEHP) and arachidonic acid esters to the substratum caused disorientation of phototaxis of Dictyostelium discoideum slugs similar to that observed with the as yet unidentified Slug Turning Factor (STF). The decrease in accuracy of phototaxis was dependent on the concentrations of these molecules. Several other structurally related lipid-like molecules did not affect slug phototaxis. Like STF, which affects phototaxis whether or not activated charcoal is present in the agar, arachidonic acid esters retained their activity on charcoal agar, whereas DEHP did not. Both compounds shifted the transition temperatures in slug thermotaxis away from the growth temperature. Unlike STF, they did not reduce the accuracy of positive slug thermotaxis. The data suggest that although none of these molecules can be regarded as a STF analogue, arachidonate and/or some of its metabolites may have a role in the sensory transduction in D. discoideum slugs.
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