A novel gene involved in phototactic motility of the unicellular cyanobacterium Synechcocystis sp. PCC 6803.

Cyanobacteria show unique phototactic motility without flagella and move themselves toward the light conditions for optimum photosynthesis. We have identified that several genes are required for assembly of the thick filaments of type IV-like pilus structure and demonstrated that they are responsible for cell motility in Synechocystis sp. PCC 6803. Natural transformation competency also depends on this extracellular structure. To further understand the molecular mechanism of phototactic motility, we attempted to identify mutations in nonmotile mutants, which appeared spontaneously in culture of the motile PCC strain. Genomic DNA of the PCC strain was fractionated after digestion with various restriction enzymes and applied to complementation assay with the nonmotile mutants. Most of the nonmotile mutants showed non-transformable phenotype. A mutant retaining some competency was selected as a recipient for transformation assay. It was found that a size of approximately 3-3.5 kbp derived from EcoRI digestion was effective for recovery of motility of the mutant cell. Finally, we identified an ORF that is required for cellular motility of the PCC strain. Detailed characterization of this mutant will be presented.