Multiple Light Inputs Control Phototaxis in Synechocystis sp. Strain PCC6803

Abstract

The phototactic behavior of individual cells of the cyanobacterium Synechocystis sp. strain PCC6803 was studied with a glass slide-based phototaxis assay. Data from fluence rate-response curves and action spectra suggested that there were at least two light input pathways regulating phototaxis. We observed that positive phototaxis in wild-type cells was a low fluence response, with peak spectral sensitivity at 645 and 704 nm. This red-light-induced phototaxis was inhibited or photoreversible by infrared light (760 nm). Previous work demonstrated that a taxD1 mutant (Cyanobase accession no. sll0041; also called pisJ1) lacked positive but maintained negative phototaxis. Therefore, the TaxD1 protein, which has domains that are similar to sequences found in both bacteriophytochrome and the methyl-accepting chemoreceptor protein, is likely to be the photoreceptor that mediates positive phototaxis. Wild-type cells exhibited negative phototaxis under highintensity broad-spectrum light. This phenomenon is predominantly blue light responsive, with a maximum sensitivity at approximately 470 nm. A weakly negative phototactic response was also observed in the spectral region between 600 and 700 nm. A ⌬taxD1 mutant, which exhibits negative phototaxis even under low-fluence light, has a similar action maximum in the blue region of the spectrum, with minor peaks from green to infrared (500 to 740 nm). These results suggest that while positive phototaxis is controlled by the red light photoreceptor TaxD1, negative phototaxis in Synechocystis sp. strain PCC6803 is mediated by one or more (as yet) unidentified blue light photoreceptors.

Asymmetric distribution of type IV pili triggered by directional light in unicellular cyanobacteria

“…To evaluate the effect of the light, we applied a mutant that lacks PixD, a bluelight-sensing protein (15,17). Unexpectedly, the T4P was generated in the ΔpixD mutant (Fig.…”

“…Phototaxis of bacteria has been visualized as colony migration on an agar plate (14,22) or as the trajectory of cells (15,18). To observe a detailed trajectory of negative phototaxis at the single-cell level, we constructed an optical setup that allowed us to simultaneously illuminate specimens on a glass substrate both laterally and vertically (Fig.…”

“…Our experimental setup enables us to evaluate the regulation of T4P filament under an optical microscope on a timescale of minutes and thus could be an ideal tool to quantify the detail of phototactic responses in Synechocystis (14,15,18,31). We also visualized the retraction of T4P by tracking a fluorescent bead that attached to the edge of a single pilus.…”

“…PCC6803, a model cyanobacteria (12)(13)(14). This species exhibits T4P-dependent twitching motility on surfaces such as soft-agar plates at speeds of a few micrometers per minute, and the cell motility direction is regulated both positively and negatively along the light axis (15). The phototactic behavior is mediated by the photo-sensing proteins and its two component systems, and, particularly at the blue-light region of the spectrum, the cell exhibits negative phototaxis (16).…”

Near-UV cyanobacteriochrome signaling system elicits negative phototaxis in the cyanobacterium Synechocystis sp. PCC 6803

“…Light wavelengths ranging from the green (560 nm) to the red (720 nm) region mediate positive phototaxis of Synechocystis. Negative phototaxis is observed in response to UV-A light (360 nm; hereafter abbreviated as UV) as well as in response to blue (470 nm) and red (600-700 nm) light at high intensity (2,3). Although light avoidance is regulated by sensory rhodopsins in Archaea and in a few proteobacterial and algal species (4), no such proteins are encoded within the Synechocystis genome.…”