Taxic responses in Phormidium uncinatum

A. N. Belozersky Laboratory of Molecular Biology and Bioorganic Chemistry, Moscow State University, Moscow 117234, U.S.S.R. ABSTRACT Summary: Cyanobacteria respond to a decrease in light intensity by reversing their direction of gliding. The sensitivity of the phototactic response in Phormidium uncinatum increased two- to threefold under anaerobic conditions. Light-dependent changes in the membrane potential ( ), 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 ( µ H ). The uncoupler carbonyl cyanide m -chlorophenylhydrazone (CCCP), when added to P. uncinatum 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 . A viscous environment, exudates of an old culture, or high concentrations of Ca 2+ (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 –6 M -Ca 2+ (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 µ H - or chemoeffectors leads to a methylation-requiring step followed by a taxic signal in the form of simultaneous changes in and Ca 2+ concentration.