Summary: Cyanobacteria respond to a decrease in light intensity by reversing their direction of gliding. The sensitivity of the phototactic response in increased two- to threefold under anaerobic conditions. Light-dependent changes in the membrane potential (Δψ), as measured by tetraphenylphosphonium distribution, were also found to be larger in anaerobic conditions, suggesting that the photophobic response is governed by sensing of the protonmotive force (Δμ). The uncoupler carbonyl cyanide -chlorophenylhydrazone (CCCP), when added to trichomes, also evoked a phobic response in a spatial gradient assay. The extent of repulsion by different concentrations of CCCP correlated with its ability to decrease Δψ. A viscous environment, exudates of an old culture, or high concentrations of Ca (plus the ionophore A23187) caused oscillatory reversals and a partial asynchronization of cells within a trichome. EGTA or CCCP in high concentrations restored synchronization. Ethionine inhibited reversals and the addition of 10 -Ca (plus A23187) restored photophobic sensitivity. A depolarizing electrical potential spread from the leading end (the “head”) of the trichomes following a decrease in light intensity. It is suggested that sensing of Δμ- or chemoeffectors leads to a methylation-requiring step followed by a taxic signal in the form of simultaneous changes in Δψ and Ca concentration.


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