The cyanobacterial phytochrome 2 regulates the expression of motility-related genes through the second messenger cyclic di-GMP

The cyanobacterial phytochrome Cph2 is a light-dependent diguanylate cyclase producing the second messenger c-di-GMP. Under blue light, the Cph2-dependent increase in the cellular c-di-GMP concentration leads to inhibition of motility in the cyanobacterium Synechocystis 6803. However, the targets of c-di-GMP in this cyanobacterium and its mechanism of action remained unclear. Here, we determined the cellular concentrations of three cyclic nucleotides in wild-type and Δcph2 cells after blue- and green light illumination. Inactivation of the photoreceptor gene completely abolished the blue-light dependent increase in the c-di-GMP content. Microarray analysis revealed that in the wild type in comparison to the Δcph2 mutant, blue light mainly led to a change in accumulation of mRNAs encoding minor pilins, putative chaperone usher pili as well as several chemotaxis regulators. The mRNA encoding the minor pilins pilA5-pilA6 is negatively affected by high c-di GMP content under blue light, whereas the minor pilin encoding operon pilA9-slr2018 accumulates under the same conditions, suggesting opposing functions of the respective gene sets. Based on mutational and gene expression analysis, we further suggest that the second Synechocystis 6803 homolog of a CRP-like transcription factor, SyCRP2, is the regulator of minor pilin gene expression and of putative chaperone usher pili genes slr1667/slr1668. Thus, our work indicates that the Cph2-mediated increase in cellular c-di-GMP concentration upon blue-light illumination specifically changes the transcriptome of Synechocystis 6803. We analyzed gene expression of an Synechocystis sp. PCC 6083 mutant strain Δcph2 lacking the unique bacteriophytochrome Cph2 (Wilde et al., 2002; Mol Microbiol 44: 04) and a control strain (WT) 48 hours after transfer to blue light (5 µmols photons m-2 s-1). Blue light induce production of c-di-GMP in the WT but not in the Δcph2 mutant strain. Green light treatment served as control where no changes of the c-di-GMP content between WT and Δcph2 mutant strain could be detected. For the timepoint (48h) we sampled biological triplicates with two technical replicates. The microarray was finally hybridized with RNA derived from two biological replicates.