CyanoTag: Discovery of protein function facilitated by high-throughput endogenous tagging in a photosynthetic prokaryote

Despite their fundamental importance to aquatic ecosystems, global carbon cycling and exciting potential in sustainable bioindustries, the genomes of photosynthetic prokaryotes still contain large numbers of uncharacterised protein-coding genes. Here we present a high-throughput approach for scarless endogenous fluorescent protein tagging in the model cyanobacterium Synechococcus elongatus PCC7942. From 400 targets we successfully endogenously tag over 330 proteins corresponding to >10% of the proteome. We demonstrate how this resource can be used at scale to determine subcellular localisation, track relative protein abundances and to elucidate protein-protein interaction networks. Our data has provided biological insights into a diverse range of processes - from photosynthesis to cell division. Of particular interest, our ‘CyanoTag’ lines enabled us to visualise in real time the rapid condensation of Calvin Cycle proteins Prk and Gap2 within seconds of withdrawal of light, effectively ‘switching off’ photosynthesis in the dark. These insights, CyanoTag cell lines, associated data and optimised methods are intended to be shared as a resource to facilitate further discoveries relevant to cyanobacteria and more broadly to all photosynthetic life.