The protein Slr1143 is an active diguanylate cyclase in Synechocystis sp. PCC 6803 and interacts with the photoreceptor Cph2.

Cyclic-di-GMP is an ubiquitous second messenger in bacteria. Several c-di-GMP receptor proteins have been identified to date, and downstream signalling pathways are often mediated through protein-protein interactions. The photoreceptor Cph2 from the cyanobacterium Synechocystis sp. PCC 6803 comprises three domains related to c-di-GMP metabolism: two GGDEF and one EAL domain. It has been shown that the C-terminal GGDEF domain acts as blue-light triggered c-di-GMP producer thereby inhibiting motility of the cells in blue light. The specific function of the other two c-di-GMP related domains remained unclear. In this study, we test knockout mutants of potential interaction partners of Cph2 for altered phototactic behaviour. Whereas wild-type cells are non-motile under high-intensity red light of 640 nm, the mutant Δslr1143 displays positive phototaxis. This phenotype can be complemented by overexpression of full-length Slr1143, which also results in an increased cellular c-di-GMP concentration. However, the non-motile phenotype of wild-type cells under high-intensity red light appears not to be due to an elevated cellular c-di-GMP content. Using co-precipitation and yeast two-hybrid assays, we demonstrate that the GGDEF domain of Slr1143 interacts with the EAL and the GGDEF domains of Cph2. However, under the test conditions, the interaction of the two proteins is not light-dependent. We conclude that Slr1143 is a new Cph2-interacting regulatory factor which modulates motility under red light and accordingly we propose Cip1 (Cph2-interacting protein 1) as a new designation for this gene product.