During evolution, nature has developed an astonishingly modular architecture of covalently linked individual protein domains. Using an array of building blocks with diverse functionalities enabled organisms to develop complex cellular networks that are critical for cell survival. The frequently observed coupling of sensory modules with enzymatic effectors enables direct allosteric regulation of, for example, second messenger levels in response to diverse stimuli. The interest in such light-regulated systems has recently increased due to the establishment of ‘optogenetics’, which refers to the concept of genetically targeting biological systems to enable optical control of cellular processes. However, the demand for light-controlled systems goes beyond that of naturally occurring photoreceptors. Even though progress in understanding concepts of light activation has been made, the rational design of synthetic tools is still challenging. Since mechanistic descriptions of light-signalling differ even within photoreceptor families, it is obvious that a more detailed understanding of the modularity of sensor-effector couples is required. To this end, we will perform a detailed study on red-light sensors linked to diguanylate cyclases. This will include naturally occurring systems with variable linker length between sensor and effector as well as artificial light-regulatable variants designed to probe mechanistic concepts of signal transduction. The identification of important signalling elements for the sensory domain and regulatory regions of the accompanying effector domain will provide insight into the modular coupling of sensors and effectors frequently observed in nature. We will perform an interdisciplinary approach combining biochemistry with biophysics and structural biology to characterise these systems. Atomic models obtained by crystallography will be functionally extended by solution scattering studies, nuclear magnetic resonance spectroscopy and hydrogen-deuterium exchange coupled to mass spectrometry to obtain structural information of elements involved in photo-activation and signal transmission. The combined results will significantly strengthen our understanding of light-signal transduction from the point of the photoreceptor as well as the effector domain. Eventually this will enable a better understanding of the modularity observed in natural light-regulatable systems and support the rational design of artificial optogenetic tools.