Phototaxis and signal transduction in Cyanobacteria

Prof Devaki Bhaya
Carnegie Institute of Plant Biology
Stanford University

101 Lecture Hall, New Biology Building
3:00-4:00pm April 24, Saturday

Abstract: Motility in the unicellular gliding cyanobacterium Synechocystis sp. PCC6803 requires Type IV pili. Phototaxis appears to be a complex and regulated light-directed phenomenon in which single cells and groups of cells participate. To study signal transduction during phototaxis we have isolated several mutants with an aberrant phototactic response (i.e they are non-motile or exhibit constitutive negative phototaxis). Several of these mutants mapped to che-like genes. Synechocystis sp has three loci containing che-like genes; all three of these (tax1, tax2 and tax3 loci) appear to play a role in phototaxis. Mutants in the tax1 and tax2 locus are negatively phototactic while mutants in tax3 are non-motile. Our analysis of some of these mutants using a novel slide-based phototaxis assay and time-lapse video microscopy suggests that there are at least two light
inputs that regulate phototaxis. There is a red light (far red light reversible) positive phototaxis controlled by TaxD1 which fits with evidence that there are two phytochrome-like domains in TaxD1. These results suggest that while positive phototaxis is controlled by red light, negative phototaxis in Synechocystis sp. strain PCC6803 is mediated by one or more (as yet) unidentified photoreceptors. Analyses of specific site-directed mutants in the hybrid histidine kinase, TaxAY1, one of the key players in the signal transduction events, will be discussed. We will also describe some of the novel mutants that were isolated in the mutagenesis screen that suggest that group behavior and signaling may be involved in phototaxis. Our working model of the complex signal transduction networks that control phototactic movement will be presented.