PHOTOTAXIS
Synechocystis cells spotted on a motility plate exhibit phototaxis as shown by the regular finger-like projections one day after growth in directional light (arrow).
To dissect the process of motility and phototaxis in Synechocystis sp. we generated a library of transposon-tagged motility-mutants. Several of these tagged-motility mutants mapped to chemotaxis-like genes at loci which we named the tax loci. The roles of chemotaxis proteins in signal transduction are fairly well-understood in flagellated enteric bacteria, but much less so in other systems. Synechocystis sp. has three tax loci, two of which are involved in motility responses. Disruption of the tax1 locus (which contains a photoreceptor, TaxD1) produces mutants that are negatively phototactic while tax3 mutants are non-motile and have no pili. Several novel mutants that are aberrant in phototaxis are being characterized using biochemical and genetic approaches. We have developed a preliminary model of phototaxis and are developing a system to analyze phototaxis in thermophilic cyanobacteria isolated from microbial mats.
Electron micrograph of Synechocystis cells showing pili connecting cells. Inset shows SEM of cells with dense pili between cells.
We have shown that in the model organism Synechocystis. sp. phototaxis is a surface-dependent phenomenon that requires Type IV pili rather than flagella. Many Gram negative bacteria have Type IV pili, which are long multi-functional, proteinaceous surface appendages. Interestingly, Type IV pili are required for diverse functions such as social motility, host-pathogen recognition, the ability to take up exogenous DNA and in biofilm formation.
Currently we are using time lapse video microscopy and tracking programs to follow single cells and populations to ask basic questions about the parameters that govern motility. In collaboration with Doron Levy (Department of Mathematics, University of Maryland) we are modeling social dynamics in surface dependent motility. We have recently also set up collaborations with K.C. Huang’s group (Department of Bioengineering, Stanford) to simulate and control surface dependent motility. It is likely that cells function as groups and dynamics of group communication may be mediated through pili and molecular signals such as cAMP. The role of communication is particularly relevant to microbial mats and other bacterial communities in natural environments.