The advent of optogenetic tools for controlling neuronal function with light has led to dramatic advances in the understanding of the anatomy and function of neural circuits. Optogenetic tools for controlling transcription and modifying neuronal connectivity could also be extremely useful for interrogating neuronal circuits. However, these tools are based on photo-activatable complexes, and all current versions of such complexes, which depend on photo-isomerization, have a small amount of background activation in the dark, which makes them difficult to implement in vivo. The experiments in this grant use a novel photo-activation complex based on the intrinsically photo-cleavable protein, PhoCl, which displays virtually no activation in the dark, and thus is appropriate for use in vivo. We will use this complex to develop novel light- activatable systems for mediating transcription and for ablating excitatory or inhibitory synapses. The latter application is based on novel technology that we previously developed that uses E3 ligases targeted to synaptic scaffolding proteins to mediate degradation of the scaffolding proteins. In turn, this results in the structural and functional ablation of synapses that they support. The final aim of the grant is to optimize the structure of PhoCl so that it can be cleaved faster and more efficiently with two photon light.