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Phormidium lacuna is a naturally competent, filamentous cyanobacterium that belongs to the order Oscillatoriales. The filaments are motile on agar and other surfaces and display rapid lateral movements in liquid culture. Furthermore, they exhibit a photophobotactic response, a phototactic response towards light that is projected vertically onto the area covered by the culture. However, the molecular mechanisms underlying these phenomena are unclear. We performed the first molecular studies on the motility of an Oscillatoriales member. We generated mutants in which a kanamycin resistance cassette (KanR) was integrated in the phytochrome gene cphA and in various genes of the type IV pilin apparatus. pilM, pilN, pilQ and pilT mutants were defective in gliding motility, lateral movements and photophobotaxis, indicating that type IV pili are involved in all three kinds of motility. pilB mutants were only partially blocked in terms of their responses. pilB is the proposed ATPase for expelling of the filament in type IV pili. The genome reveals proteins sharing weak pilB homology in the ATPase region, these might explain the incomplete phenotype. The cphA mutant revealed a significantly reduced photophobotactic response towards red light. Therefore, our results imply that CphA acts as one of several photophobotaxis photoreceptors or that it could modulate the photophobotaxis response.

Phormidium lacuna, a filamentous cyanobacterium without heterocysts, can be transformed by natural transformation. These filaments are motile on agar and other surfaces and display rapid lateral movements in liquid culture. Furthermore, they exhibit phototactic response under vertical illumination in Petri dishes. We generated mutants in which a KanR resistance cassette was integrated in the phytochrome gene cphA and in various genes of the type IV pilin apparatus. pilM, pilN, pilQ and pilT mutants were defective in all three responses, indicating that type IV pili are involved in all three kinds of motility. Rapid movements of wild type in liquid culture requires an extracellular matrix with type IV pili as central player. pilB mutants are only partially blocked in their responses. pilB is the proposed ATPase for expelling of the filament. In the mutant, this function could be overtaken by an alternative protein, like pilT, which regularly mediates retraction of pili. The cphA mutant revealed a significantly reduced phototactic response towards red light. We assume that together with other photoreceptors, CphA regulates the phototactic response by down regulation of surface attachment.